I recommend the Didj from Leapfrog. We got one for our son when he was 5 and he LOVES it. He hated the Leapster because all the games were way too easy for him by the time he was at the end of kindergarten. The games that listed "K-2nd" was all stuff they taught in kindergarten! My 4 year old doesn't like the Leapster, either. One reason for both is that they don't like the bad graphics. And even she is bored with many of the "prek-k" games (basic numbers, letters, letter sounds -- all stuff many 4-5 year olds already know before K).
Anyways.... our oldest daughter just turned 4 a few months ago and she's been swiping his Didj to play with it. And she loves it. Again, many of the games that say "K-1st" is stuff she's already learned in pre-school. Leapfrog has a warped sense of what is taught in each grade level (as myself and many of my mom friends have discovered). She's playing many of the "K-2nd" grade games. We're planning on getting her one of her own this Christmas.
The Didj is nice because it will last for many years. They have games that cover 4th and 5th grade level stuff. My son is in 2nd grade and uses his almost daily. Every Monday, I hook it up to our computer and give it his weekly spelling words so he gets extra practice with them when he plays a grammar/reading game. I also customize the math games by entering the current skill they are working on in class (like times 3 this week). There aren't as many games available as the Leapster (only about 15) but I am told many more are in development, especially for upper elementry and jr high grades.
One thing to note is that the Didj is not like the other educational systems out there. The actual games themselves are not educational. They are normal video games with good graphics and action like racing or shoot 'em ups. At certain breaking points in the gameplay, it switches to a quiz screen where you answer 5-10 questions. After you answer them the game continues. The more correct answers you get, the further you get in the level. Correct answers also unlocked new levels, hidden rooms, 'buy' you extras, etc etc so there's real incentive for a game loving kid to answer correctly to get 'stuff'.