Ooh, this is a fun question.
Jannah, simply put, is this:
"They will have whatever they wish therein, and with Us is more." (Quran 50:35)
What a simple answer, but to go into more depth, is the following. The residents will get whatever they wish, because that is God's awesomeness for you. But if you look more closely, there are sorts of hidden rewards as well. Such as the second part of already stated verse. Or in the following:
"And no soul knows what has been hidden for them of comfort for eyes as reward for what they used to do." (Quran 32:17)
Now, we have to use some thinking here. If everyone keeps everything they know of, what is the benifit of going upper into the layers? I think its more of the hidden rewards.
"Abu Huraira reported Allah's Apostle (ﷺ) as saying that: Allah the Exalted and Glorious, said: I have prepared for My pious servants which no eye has ever seen, and no ear has ever heard, and no human heart has ever perceived but it is testified by the Book of Allah. He then recited:" No soul knows what comfort has been concealed from them, as a reward for what they did". (xxxii. 17)" (Sahih Muslim 2824)
You see where I am going with this? You get everything you want, and that will be available for everyone, but the real incentive are things so much cooler than that. And this is where I dwell into psychology. If you don't know what it is, how can you be jealous! There was a hadith as well, that stated that such unknown things are even better than the ones we know of.
I am not going to go much further, but I am going to give you the links so I don't have to type it all. It is just so numerous of the stuff we already know of. Yes, there are castles, and not just ones of stone, but of gold! Milky rivers, a tree with 70 years of shade, pebbles of pearls, and other stuff. A space of a whip of heaven is better than the entirety of earth. No aging, no spitting, all humans will be 90 feet. Yes that is a thing. So it is too much stuff for me to state. Here are the more indepth links:
If you want more, you can find endless stuff in the hadith.