Check out this site:
http://orgjunkie.com/2008/06/rounding-up-the-playroom.html
But I'm not posting anything the way it looks now (My Little Pony explosion)
We have two big rooms, plus the storage side, in our basement. One of the finished sides is their playroom. We lined one wall with bookshelves, another wall had the kitchen playset, dress up clothes storage. The third wall has two big magnetic white boards, and then their computer armoire fits into this one area.
In the middle is a big ol' 4 ft round kindergartener size table (was actually an old school table so it is HEAVY) that I bought off of eBay, and some plastic school chairs.
Give them lots of shelves. We have two 3 shelf units (Sauder probably) sandwiching matching long lowboy that has two shelves in each section.
Get some smaller plastic containers/bins/baskets to contain stuff. I like those fabric drawers, but you're putting crayons, pens, etc in them, the plastic hold up much better. I only put the fabric in their rooms and closets.