Re: Greetings! Sooner fan visiting Iowa for the first time!
Some good suggestions so far. Not sure if you're checking in during the week, but I'll take a chance you're still checking in on the forum and provide some suggestions.
For overall activities going on in the metro, check out www.seedesmoines.com.
For more downtown related info, see: www.knowdowntown.com
Johnston is one of the nice suburbs in the metro, but pretty boring if you're 26 and trying to get a feel for our metro. I would agree with others, you should check out downtown.
If you have free time during the day, take a tour of the Iowa State Capitol Building. It is very impressive structure given our modest little state.
If you like pizza and would like to try some varieties that are only found in DM, be sure to hit Fong's Pizza. Fongs Pizza
I have other suggestions, but will wait to see if you're still checking the site (or if you want addl suggestions...don't be afraid to be very specific on what you'd like to see/experience).