M.M.
We finally decided our soon to be 6 year old was old enough for a DS for X-Mas and she would receive while on a trip in Florida. I brought our leapster for our 3 year old to save and potential fights. Wouldn't ya know it, little Miss Three year old wanted NOTHING to do with the Leapster and was actually pretty good at the DS. Day after X-Mas we were out buying yet another present for the princess. Soooo worth it!! There are games that involve no reading, but have matching, coloring, musicical instruments....
Smart Girls: Playhouse & Smart Girls Party Time (I think this is the other one) are a huge hit with my kids.
Never thought I would say this, but I think a DS for a 3.5 year old is a good investment....especially if you are traveling. Great for going out to dinner too :)