I like both our pools in Elmhurst. The Smalley Pool is zero entry, which seems to be getting more common in the suburbs, so it might not be that unusual. There are also a couple of fountains and things that my two-year-old really likes. The East End pool has fountains in the baby pool.
Elmhurst also has a spray ground at The Hub. I don't know if you'd get a day out of just the spray ground, but we usually get a morning out of it.
Here is the link to the pools. http://www.epd.org/pools.html. Once you get on this site you can also find information about the hub. You can go to any of our pools as a nonresident; it just costs a little bit more.
Another suburb that has a good pool is Naperville with their Centennial Beach, which could be a nice day outing because it's a beach setting with a zero entry pool and also has nice parks, restaurants, and shopping within walking distance. They also have the dragon sculptures in town right now, which my kids love.
I understand Lisle also has a fun water park - Sea Lion or something like that, but I've never been to it. You could probably check out Lisle and Naperville park district web sites to get more information on their pools.
Good luck and have fun!