Few years ago, the Wife and I did a week vacation. We didn't want to travel a huge distance and wanted to do something different too. We don't have kids at home any more either, and just our pets. I'm a model railroad buff, so this one vacation we decided to break-up the camping. One campsite in Southern Indiana, one in the Central Western area of Indiana, and the third in the Northern area of Indiana. Reason? We did an internet search and discovered that Indiana had 7 different railroad museums and several of them had working railroads (engines and passenger cars). We'd already rode the one in Connersville and Noblesville Indiana, so decided to hit the other 5 ... which we did. We stayed 3 nights at one location, moved to the next for 3 nights, and then the last for 3 nights, and then headed home. It really was a great vacation and we had an absolute blast.
So, if you do some research, and locate the various railroad museums in your area, you might consider doing something like this. If your not a railroad "buff", you might consider something else? Do you have interest in minatures (like doll houses), or how about exploring the book "Weird Minnessota" (if it exists.
For what it's worth, the wife and I are now focusing our vacations (still in Indiana) based on the book called "Weird Indiana". We're selecting campsites (state parks usually) within proximity of something talked about in this book and then go search it out. So far, it's working out great! It gives us a destination point and something to do, and includes a little research into the area or nearby town.
We've done the hiking, swimming, bicycle riding stuff when the kids were little. Now that we're a little older, I guess we've journeyed into the "weirder" side of life. .... but hey .... we're having fun doing this ... something to consider!
Have fun!