May is a really pretty time of the year in the Midwest. The trees look like someone has just painted them with a velvet green brush and the colours are very vibrant. We are having a very oddly warm winter - temperatures at least 10-15' warmer than normal, so spring will either not happen if we get hammered with a late winter, or will arrive much earlier than usual. May temps usually can vary from needing a down jacket & gloves to trying to fit into last summer's mysteriously shrunk shorts
Drop me a pm when you are heading towards Oshkosh. If I'm home will definitely try to meet up, if you want

Best hotel is the Hilton - right on the airport. If you come over the weekend, Pioneer Airport will be open
AirVenture Museum - Pioneer Airport It doesn't open weekly until the end of May.
As for hotels... I stay in a quite a few over the whole country on quite a regular basis. The quality of the hotel depends very much on who is running that particular one, not necessarily on the chain/brand name. A Best Western in one town can be fantastic but in another, it will be less so

. We usually use Hampton Inn, or Country Inn & Suites - they tend to be the nicest without being a crazy price. Staying in MSP recently, the hotel (Marriott, I think) was perfectly nice, but the rooms were tiny in comparison to many I've stayed in. I think it was something to do with being in the area of the Mall of America, where space is at a premium so they are fitting in as many as possible. The Marriott hotels tend to charge for internet. There are also Bed & Breakfasts in some places. Usually very nice, and the hosts are always extremely friendly, and very proud of the history of their home. Google the town name & B&B to find a list.
If you are going to MSP, the surrounding airports have great aviation history & aircraft hanging around. Take a trip to Anoka (KANE) on the north side of town.
The People and Planes of Anoka County | History of Flight | Air & Space Magazine Greg Herrick keeps his collection of unique & beautiful aircraft there. It isn't a public museum & may take a little advance planning, (the website suggests emailing in advance to set something up) but I can guarantee you won't be disappointed with your visit - it will be well worth the extra effort. His Tri-Motor is restored immaculately, polished aluminium and just beautiful.
Home KSGS, South St Paul is where Wipeline Floats are based. As well as a CAF group & some great old hangars.
The EAA calendar, if you are interested in visiting aviating folks along your route, is great for finding out where there are pancake breakfasts & fly-ins. May is normally when flying in the Midwest really starts to come out of winter hibernation, tho' this year has been extraordinarily mild. You could also post a "Hello, I'll be in the area" on the EAA Forum - you never know who might offer you a ride or have other suggestions on places to visit.
EAA - Aviation Calendar of Events
EAA Forums
I've landed at Rapid City twice. Flying in, it looked like fabulous country to go horse riding in - miles of open space & very few fences. Very dramatic scenery to the west of town, and no shortage of fascinating places to visit.