People do some amazing things! Here are some links to before & after pics and descriptions of the fabulous stuff they did.
Check out the two level book nook, play table that rolls into the shelves, and the pretty wood desk in Jessica's Playroom from Juicy Bits.
For non-Ikea whimsical furniture and lighting ideas see Alicia Stanley (Nothing against Ikea! I just love things that are a little different than the mainstream.)
Another good idea for a craft table with plexiglass and fun paper.
Tip Junkie's collection of playroom ideas.