Lake Rathbun
Places to stay:
Honey Creek Resort - on the lake, restaurant, indoor waterpark for the kids, golf course
Foxtail Cabins - not on the lake, but they have a fishing pond
If you look on AirBNB, there are cabins east of the lake across from the marina. Not sure what the name of the place is, but here's a link to one of the cabins:
https://www.airbnb.com/rooms/923914231879219831 They have a 6 acre lake with a swimming beach, kayaks, etc.
Things to do:
Centerville - downtown area has some shops
Davis County (Bloomfield area) - Amish (Davis County has the largest Amish population west of the Mississippi). If you've been to the Amana colonies - don't expect it to be like that. More info and directory:
https://www.visitdaviscounty.com/amish-community
American Gothic Museum (1 hr 15 min drive from Rathbun) - most people don't realize the American Gothic house is in Eldon, Iowa. The house is privately owned. The museum is located across the street. There's an area you can stand for pictures and the museum also has clothing you can put on so it looks more like the painting.
Hannibal, MO
Not located in Iowa, but less than a 4 hour drive from Des Moines.
Mark Twain Lake is located SW of Hannibal - about 45 minute drive. Hannibal is the home town of Mark Twain and Molly Brown (Titanic survivor). There are a lot of sites related to the Tom Sawyer books - the fence they paint, the cave Tom & Becky are in. Other things - a light house, riverboat cruises, etc.