CWS is big in the summer, though I never made it over to see any of the games before we moved south.
Omaha usually gets decent concerts during midwest tours. 3hrs from Des Moines for the midwest tours that don't hit Omaha.
Omaha usually has good airline fares - enough 'better' for people to drive 3 hrs from Des Moines to fly out of Omaha.
IIRC, there are casinos on the Nebraska side of the river.
Loess Hills are on the Iowa side of the river - good for hiking, camping, etc.
Hop-skip-and-jump from Lincoln for fall football.
Potential for LOTS of snow in the winter. Potential for hot and humid in the summer (at least by midwest standards).
Not 'too' terribly far from Black Hills for summer vacations.