I was going to recommend the zoo, too. This is a lovely time of year. You might also check out some local hiking trails and do a morning hike.
If you go to the Botanical Gardens be sure to take some photos of your kids next to the giant cacti!
7yo: "Mom,I know which man is on a quarter and which on is on a nickel. They both have ponytails, but one man has a collar and the other man is naked. The naked man was our first president."