A couple of years we ago, we made the kids a Funfort in their room (see howto here.). It was awesome and the kids loved it.
The only downside was that it was pretty big and took up quite a bit of the space in their small room. They started bringing more and more toys downstairs to be somewhere with more area to play. So, with their permission and approval, I took it all apart and converted all the lumber into shelving for around their room.
The room has now been rearranged to fit the new furniture. There is plenty of space for all their toys (and, of course, all the toys we inherited from Abe’s childhood too!). They have joint shelves and individual shelves, so that Leo can keep his stuff as organized as he wants, while Nicky can have his as messy as he wants.
It should work fine until we get to build our room on the west end of the house (whenever that might be), at which time the kids will have a room each.