A boy once went out with his Dad for a day of fishing.  They sat there all day on a boat, and caught nothing.  Upon returning home, they both privately wrote in their journal entries for that day - summaries of the day.  The Dad wrote in his journal, "Took my son out fishing.  Caught nothing, day wasted."  But the son wrote, "Dad took me out fishing.  It was the best day of my life."
It's easy in all our busyness and obligations of life, to miss what's most important in this life and slowly become blind to it without noticing.
I've noticed that as long as I've lived, one thing has been a constant: there is always much to do.  When I find myself with free time my mind naturally tends toward thoughts such as "What can I do next, how can I get more done? How can I make a dent in many of the outstanding items on my task list?"  Another thought that often comes is "What can I do to pass the time now?  What enjoyable activity can I do?"
I've tended to think the first is more spiritual than the second, and sometimes found guilt in defaulting to the second.  I've recognized our life has to be balanced - Martha was corrected for too much of the first (to the neglect of sitting at Jesus feet), and 1 Corinthians 6:12  says, "All things are lawful for me, but not all things are profitable. All things are lawful for me, but I will not be mastered by anything." (we are free to do lawful things - they are good gifts from God to enjoy freely (1 Tim 6:17), but I should never be mastered by them, holding them as idols).
So the question I've come back to again and again is: how do I spend my time in the way God wants me to?
I've been challenged by a picture of Jesus in Matthew 13:1
"That day Jesus went out of the house and was sitting by the sea."
To the world, and even many Christians - sitting down on a beach looks like a waste of time.  Is Jesus relaxing His life away when the world is perishing?
Was this one of those 'lawful but not profitable moments' for Jesus?
No, I believe the most profitable thing for Jesus in that moment was actually to be by the sea.  And in my mind - I see Jesus content to sit there, spending time with the Father.  Having nothing in front of Him, nothing to do, no pressing task on His mind.  Just having some alone time with His Father, not worried and anxious about so many other things.
This moment of Jesus' life has often refreshed me.  I don't have to hold the world on my shoulders, I don't have to micromanage every ounce of time - in fact, that is the thing that can make me waste my life altogether, just as the Dad missed the value of a day on a boat with his son, thinking the point of the day was 'productivity'.  The main thing is: devotion to Jesus.
One thing I've learned about this, which has been a practical thing for me is recognizing that the physical action (i.e. spiritual activities) I'm doing with the time I take with God is secondary.  It doesn't have to look the same all the time, based on a 'law' or rule of 'this is how you spend time with God.'  Sometimes maybe it's sitting at Jesus' feet with my Bible, sometimes it's with nothing.  Sometimes it's meditating on a verse, or listening to a sermon.  Sometimes it's talking to God about some pressing burden (I remember times where I'm tempted to overwhelming anxiousness and I've sat down with the Lord for just 1 or 2 minutes and He completely lifted me out right then)  "Cease striving and know that I am God" (Psalm 46:10).  Sometimes it's saying thank you to God for all His blessings, and "counting my blessings" out.
Many times I've had to slow down and not be in such a hurry and rush to do everything which I think I need to do, because not all of it may be what the Lord wants me focusing on right now. "..Martha, Martha, you are worried and bothered about so many things;  but only one thing is necessary.." Luke 10:41-42.
And also - much of my 'hurry' can be driven by worry, and a desire to control things myself.  The KJV, in the end of Isaiah 28:16 is talking of Jesus, and says "he that believeth shall not make haste."  I heard a paraphrase of it which blessed me: "He who trusts in the Lord will not be in a hurry."
I heard a story of a man who gave another man a warning: "Hurry is of the devil."  To which the other man replied, "No. Hurry IS the devil."  Without trying to analyze that to see if it is Biblically correct or not, I get what he was saying.  Because "hurry" (caught up with many anxious distractions) has often been the very thing to draw me away from God, just as Eve was drawn away from God (2 Cor 11:3).
I want to be careful how to say all this, because the amount of free time in the day is very different for everyone.  And some people need to move quicker than others to get their jobs done!  It's a mindset - our mind should be free from unrest, and fretting about the future, that's what I mean.  We should be at rest in our mind, from worry.  Secondly, many people don't have a lot of chances to sit at a beach talking to God.  Sitting alone somewhere talking to God is not the definition of what it means to 'sit at Jesus feet'.  It's not an outward thing.  Even here in Matthew 13:1 it says, "THAT DAY Jesus went out..", which to me implies - He didn't go out ALL the time for alone time with God, but He did sometimes.  Not everyone has a 'secret room' in their house either, where they can get alone with God.  But we all have a 'secret room' in our heart where we can retreat with God ANY time - secretly being with Him, no one else seeing it.  Praise God!  What a precious room we have!  If only we will use it.
Likewise - not all time of rest which is not 'spiritual' (eg in the Bible, or specifically in prayer) is not a waste of time either.  Even Jesus called His disciples away for plain and simple rest: "And He *said to them, 'Come away by yourselves to a secluded place and rest a while' (For there were many people coming and going, and they did not even have time to eat)" - Mark 6:31.  Giving our bodies and even our MINDS rest is profitable!  As long as it's in the right balance (not being 'mastered' by anything, as an idol).  This has been a very freeing truth or me, since temptation to guilt in this has been a common struggle of mine.
But the point which I've found and been blessed by in seeing Jesus there on the beach at His Father's feet, and I want to emphasize is two-fold:
1. I want the heart like Jesus, where even if I don't have free time - I have a longing after God, where I often am eager to spend time with God.  And where even if I don't have much free time, I can be in the "secret room" of my heart with Him, regardless of whatever I'm doing, talking to Him in the midst of the busyness of life.  ie "Pray without ceasing" (1 Thess 5:17).  And when I set aside specific free time with Him, to remember this:
2. Plain and simple spending time with God is never a waste of time - it's really valuable time, eternally invested.  Just as a date with my wife is not a waste of time, and sitting down with my kids to play is not a waste of time.  The value of my time I spend with God will definitely last forever, and bless the Lord's heart now (isn't this the greatest blessing of all?), as He wants dearly to spend time with me (what a humbling thought!).  The Father is rejoicing to see His children at His feet looking up at Him - God has a heart full of joy to see us there often (Zeph 3:17).
So, in conclusion: spend time with God.  Whenever it is, or whatever it looks like - have a heart which is drawn to God.  And if you're like me, as you're reading this sentence, you may say "Well that's the problem, that's my exact struggle - keeping a heart which is drawn to God."  But just as we were saved by God's power through faith, we are preserved through it (Gal 3:3).  He started the work in my heart to draw after Him, and He has to finish it.  He has to keep the fire stoked.  So for me that means I've found rest in: 1. Being HONEST ("Lord my heart drifts so easily, forgive me!"), 2. Asking for HELP ("Lord put a fire in my heart for you always"), and lastly,
3. BELIEVE that God will do the inner work, which I can never do:
He will surely not ignore a sincere cry like that in a child of His who wants to be more devoted to Him - that's impossible.  He WILL answer.  He ALWAYS fills those who are hungry, and ask/seek to be filled (Matt 5:6, 7:7).