Raccoon River park & beach in West Des Moines is about a 10 minute drive from our house, and I've gone with the kids a few times now. The playground is pretty big, which isn't a good thing when there's a beach in running distance. I have to make sure I always know where the kids are. When Andrew comes with us, the trip is definitely worth it. The kids love all of the equipment, and they really, really love the beach!
And then, like the responsible parents we are, we had the kids run through the splash pad to get sprayed with chlorinated water to clean them up for the beach :)