So we've been to St. Louis a ton, as my husband has family there. Magic House is perfect for a 2.5 year old; we just took our kids there in October (ages 3, 5.5, and a baby) and they could have spent all day there. City Museum is very neat and they do have a toddler area, but might be better for a slightly older kid. We also did the aquarium on the last visit, and the kids loved it but I thought it was a little expensive for what it was, as we only spent about 1.5 hours there in total. The Science Center and Zoo are both free, though the Zoo is mostly outdoors. Also, the Missouri History Museum in Forest Park is also free, and has a good children's exhibit. For food, Ted Drewes is awesome, and St. Louis has wonderful Italian food in addition to surprisingly good Chinese food (Cate Zone for Szechuan, and we always do Lulu Seafood and Dim Sum for cart-style dim sum.) Also worth tracking down St. Louis Gooey Butter cake. Enjoy the trip!