There are a number of different questions here, and though they are all related, they could probably make multiple good questions. That being said, I'll try to answer them. One thing to note is that we know very little concrete facts about heaven, so most of this is simply inferred from Scripture.
Is it possible that after reaching heaven, you find yourself discontented? Like what if the things that make you happy are not there?
This is a very popular question (just look at the number of google results), but I would say that the answer is bound up in the answer to the first question of the Westminister Shorter Catechism:
What is the chief end of man?
Man's Chief End is to glorify God and enjoy him forever sums up the answer to your question nicely. If the purpose of our existence is to glorify God and Enjoy him forever (See 1 Cor. 10:31, Psalm 73:24-26), then we need nothing else in Heaven. Indeed, in heaven we will worship God, and as such our chief purpose will be fulfilled, therefore, we will be satisfied. Additionally, you could look at it from the perspective that since discontentment is sin (basis found in the Tenth commandment), and there is no sin in heaven, then there will be no discontentment in heaven. (for the broad answer of "What will we do in heaven", see here).
Is it possible to go to hell after deciding you don't like heaven?
The Westminster Larger Catechism, question 86 -
What is the communion in glory with Christ, which the members of the invisible church enjoy immediately after death?, states:
The communion in glory with Christ, which the members of the invisible church enjoy immediately after death, is, in that their souls are then made perfect in holiness, and received into the highest heavens, where they behold the face of God in light and glory, waiting for the full redemption of their bodies, which even in death continue united to Christ, and rest in their graves as in their beds, till at the last day they be again united to their souls. Whereas the souls of the wicked are at their death cast into hell, where they remain in torments and utter darkness, and their bodies kept in their graves, as in their prisons, till the resurrection and judgment of the great day.
The key thing to note here is that
their souls are then made perfect in holiness. As I note later, if their souls are holy, why would they want to leave the presence of the supremely holy God?
The only people I think that might argue this would be those who argue you can lose your salvation, and I think even most of them would say that once you are in heaven, you are there to stay. But your question was not if you could get kicked out of heaven, rather if you could voluntarily leave. This is tied into your third question:
Do we still have free will?
That depends on how you define free will. I am from a Calvinist theological background, and so I would say a resounding yes. Here's why, and I'll give an example to explain it:
Many find the concept of free will impossible to reconcile with the concept of predestination. But tied into this is the concept of Irresistible Grace. That is, when God calls you, it is so good you cannot resist the pull. Think of a wonderful chocolate cake (or some other delicious food). When you smell it, when it is sitting right in front of you, in a sense it is irresistible to you in that you will eat it because it is so good. God is so much better, so that when he offers himself to us, we are irresistibly drawn to him because of his infinite goodness. The same applies in heaven. We will have free will, but our eyes are opened to God's goodness, and as such we freely make the choice to worship him. As such, we would not want to leave heaven, and we would be completely content. (if you reject the Calvinistic argument I make, the rest of the argument I believe still holds)