Visit lovely South Des Moines, hit up Tasty Tacos and then down the street to Romantix...
Just kidding.
I've gotten some good deals off the kind of places Serate mentions, like $42 a night at the Renaissance Savory in downtown Des Moines. That was a few years ago though, so who knows. That hotel does not have a hot tub, but it's very nice and in a good location if you wanted to take the ol' lady for dinner and drinks downtown.
In Des Moines if the weather holds up, I would suggest taking her to the Art Center during the day for lunch. For one, it's a nice place with many great pieces of art. Secondly, they have a cafe that serves some fancy food at reasonable prices in a nice environment. Eating outside by the reflecting pool on a nice day, that's good times, my friend. Even if the weather doesn't permit being outside, it's still a good bet.
Anyways, they have a soup, a salad, a sandwich and a pasta entree. Usuallly the food is somewhat innovative and at $8.95 you can get one thing or a combination of two smaller portions (say, half a sandwich and a small salad). They use high quality ingredients and I've never been disapointed.
Just shooting from the hip here, I'd say that the Art Center's restaurant is the best dining value in Des Moines considering what you get.
D M A C :: V I S I T
The building is adjacent to Greenwood Park, which would probably be worth a quick stroll-through. During the summer it's full of roses and stuff, but probably not now.
On W1ngzer's idea, Grey's Lake just south of Downtown on Fleur would be a good place to go for the sunset, city lights, etc. Grab some coffee from Cup o' Kryptonite down the road or something to bring with... but not Java Joe's because they hate punk rock.
If you do decide to go to Des Moines and need some sort of dining recommendations, I could help you with that. There's all kinds of good restaurants with reasonable prices serving all kinds of food.