We planned our basement to have a large room for toys and space to play. Toys either lived in our family room or were dragged into the family room from the kids' bedrooms so we wanted a place for them to just live. It's carpeted so the floor is comfy to kneel and sit on and all the kids toys and books are there. We also put our wraparound couch down there. We left a space for a kitchenette someday and installed our old fridge down there as well. It's so nice to have extra fridge space. We go through so much milk it would fill up our entire fridge so we keep a couple of gallons upstairs and the rest downstairs to grab it when needed. And when we have parties and dinners there's plenty of room for all the food. It's been such a nice thing!
The bookshelves are Kallax (I think one is an older Expidit which that Kallax replaced) from IKEA and the bins are all from IKEA, too. I regularly go through and give away stuff the kids don't play with anymore. In my experience, the ones that stay are cars of all shapes and sizes, big building blocks (we love a Melissa and Doug set but tried a few other sets with smaller blocks and they were constantly dumped on the floor but never played with so they were donated), Legos (regular and Duplo), Nerf guns, Lincoln logs, and dress up clothes.
I try to collect mostly hardbound and board books for the kids. The paperbacks never last very long. We do have a bunch, though, and they're kept in the blue bin. The rest of the books are arranged by color. I used to organize by author/series and size but the kids never put them back where they belonged since they couldn't read at the time and it always looked like a mess. I was a little doubtful I'd like arranging by color because I thought the different sizes would make me crazy but was happily surprised to find I love it. The kids can put them back much easier and the differing heights doesn't bother me at all. Seeing the rainbow of books makes my nerdy heart happy.
Dan and I talked a lot while designing about how we wanted to use this space. Dan had plans for a ping pong table and foosball table and stuff and I was really reluctant because I didn't want more crap in our house but we both agreed that when our kids are older we want our house to be the one our kids and their friends come to after dances or to hang out so we need to make it a fun place to be. This big space with a comfy couch and fun games and things to do helps facilitate that so I acquiesced. Even with the tables there's enough space to build things and play.
We've rearranged the room a couple of times since moving in but this arrangement seems to be working well. When we have a fire it's nice to have the couch around the fireplace. We also do our family scripture reading and family prayers here at night and having it sort of walled off from the rest of the play room helps cut down the distractions so we can focus. We have a couple of walls wired for TV but we only have one TV and plan to keep it that way so it's just a room for playing. So far it's been a really good use of space for the way we live.