S.,
Watch in the paper or online or simply call the local library as they offer some really fun things in the summer for mostly your older child but sometimes for little ones too. The free splash parks is a really good idea, they are fun, free and you end up meeting some really nice people (many times in the same boat as you!) while the kids play. You might even consider finding someone who has a child similar in age to your youngest one to set up playdates with. You watch her child for 3-4 hours on a Saturday (the kids get to play together) and she does the same thing for you the next Saturday. Kids don't really need a lot of "activities" as much as they need to be able to play, learn and interact with each other. I know it would be a long drive, but there is a really nice play structure at Lake Zorinsky and a great lake to walk around, if your child will sit in a stroller. Take lots of buckets and shovels and your child will dig for hours.
If you ever feel like taking a longer drive, the Children's Museum in Lincoln is absolutely awesome! Even though we live in Omaha, I bought a season pass to the one in Lincoln. It is worth the drive!
Hope this gives you some good ideas.
C.