I always recommend checking out the seedesmoines.com for ideas. Here is what is on the calender for Saturday:
Calendar - Greater Des Moines Convention and Visitors Bureau
A few standout IMO.
Cinco De Mayo street party in the East Village. This might work out well since you mentioned they might want to see the sites. This would put them within walking distance of the Iowa State Capitol Building & grounds.
I-Cubs are in town. 7:05 1st pitch.
Also, even non-artsy fartsy types tend to like the Sculpture Park, so that is worth a look.
For all the above, I'd recommend parking somewhere downtown and getting B*Cycle bikes to get around (assuming you have decent weather).
Des Moines B-cycle | Des Moines Bike-Sharing Program | B-Cycle | bcycle.com You could take the trail past Principal Park and around Gray's Lake and back. Note: I think it is something like $5 per bike for 24-hr access (ignore the membership plans on the website).
Here's a decent map that shows some of the bike trail options around downtown but is also helpful in general for downtown sites.
Downtown Des Moines
Also (even though it seems to be in never ending construction mode), worth heading down to the Des Moines River and checking out the riverwalk.
Tons of great places to eat downtown, I'm sure people would have recommendations if you need them.
