Des Moines hosts a farmers market downtown every Saturday in the nicer weather months, including August! Several city blocks worth of street is full of all kinds of local vendors, we visited often when we first moved here and will be getting back once it opens back up!
Highly recommend as a Saturday morning activity. If you're staying downtown, you may even be able to walk to it instead of having to drive and pay to park!
Downtown Des Moines is very walkable, and has bikes around to rent! There's the riverwalk (including unusual bridges and a chinese garden) and lots of different shops to check out! I added a link to a site with a lot of recommended places.
We did this when Ben's parents came to visit us in October! We did the most standard trip, just a nice train ride to see the area. They also have dinner/lunch/picnic trains, and if you're more adventurous they have the Rail Explorers thing where you pedal your way down the track.
Smash Park is a bar primarily featuring Pickleball, and also has things like Axe Throwing and Trivia Nights. Ben and Kennedy have come here many times with friends, including axe throwing, "Mean Girls Trivia", and a work event playing pickleball and "bags" (Iowa calls Cornhole/ Corn Toss "bags" for some reason).
Des Moines has a minor league baseball team, the Iowa Cubs, who has a game the Friday night before the wedding!
We looked at the zoo as a potential wedding location! Not a huge zoo, but we had fun checking it out.
We've visited a few parks around town that we enjoy!
Ledges State Park- North of town not far from Boone, we did some hiking at this state park when Ben's parents visited.
Easter Lake Park- South East Des Moines, by our house
Gray's Lake Part- Central Des Moines, we attended Des Moines Octoberfest on the west side of the lake!
Saylorville Lake- North of town, not far from John Deere where Ben works. Biggest lake around, cool road goes right across it low to the water!
Pots and Shots is a plant themed bar in West Des Moines! Kennedy has gone there multiple times for classes on special plants and potting. Place has drinks, some food (more like appetizers), and plants!