My one and only child (daughter) is 12. When she was about 4, we bought a large but portable Casio type keyboard. It had all kinds of buttons and whistles to make sounds like other instruments or noises and had drum buttons. It was fun then for her to poke around on the keyboard and make a lot of noise.
We still have that keyboard in a game room. We have noticed when she has friends over, we hear them playing with it. Through elem school and present, she has had music classes and she will just go play on that keyboard herself as well.
At the time, we over bought (age wise and money wise) but it has been a great investment, entertainer, and learning tool.
We found that if we bought a little ahead of her she would grow into so many toys. I think Leapster was just getting started when she was moving out of that stage. She had a little Vtech laptop (that still works). At this point, she has her own real Dell laptop which we have also found to be necessary for her school work and not just a nice thing to have.
Good luck!
Susan