Des Moines native here- The Embassy Suites is an awsome place to stay if you are catching a concert at the amphitheatre. The hotel literally opens up to the theatre and I believe the hotel staff even serve the alcohol for the concerts. Great place to catch a show, you are basically sitting on the river bank watching the band. You may want to bring a blanket or a folding chair unless you don't mind sitting on the grass.
The Iowa Cubs (AAA) have a beautiful stadium and it is very close to many good restaurants and bars. I have to say that IntheWater did a great job of recomendations. The High Life/El Bait Shop are usual stops for the baseball fans. If you hit the High Life, I love their bacon wrapped tator tots dunked in ranch. Their entire menu is great old school stuff (sloppy joes, deviled eggs, hot beef sandwiches). Its all very reasonably priced.
Another place very close to the stadium that can be a lot of fun is the Hessen Haus
www.hessenhaus.com. Enjoy Jaegerschnitzel and Reuben Rolls and wash them down with a Boot of German Bier. Tough to beat a good polka band on a Friday night.
I'm not sure you what days you will be here but the Des Moines Farmers Market
http://www.desmoinesfarmersmarket.com/About.aspx every Saturday from 7am to noon downtown is a must. Hundreds of street vendors selling all types of foods prepared and not. Prepared foods from the various ethnic communities El Salvador (Papusa's), Chinese, Tai, Indian and more. Fresh baked artisinal breads, local wineries, cheese makers, and usually couple of bands playing for entertainment. Plus all the Iowa raised meats, produce and plants.