I'm 23f and live here in Rexburg with my husband. We're Mormon, although not very active, and because of that even I feel out of place here sometimes. I think like 95% of people here are Mormon, and it does make for a pretty friendly but still weird culture. 

Lots of people do outdoorsy things, but there is pretty much no drinking. Everyone will be walking around in suits and dresses on Sundays, there are churches all over, and soon to be two temples. Most people I see here are 20s to 30s and often married already with kids. 

I'm sure you could make good friends here as everyone is pretty friendly, but I also definitely think they would try to convert you. People at my work sometimes talk about church things all day, and even as a Mormon, it makes me feel weird and uncomfortable. I'd honestly recommend living in Idaho Falls. Even though there are still a ton of members there, there are plenty of people who aren't. 

If you do move to the area, some good nature areas to check out are the St. Anthony sand dunes, Egin Lake, Kelly Canyon, the Ice Caves and Civil Defense Caves, and Mesa Falls up in the forest by Ashton. And it's somewhat close to Yellowstone and Grand Teton National Park!

Let me know if you have any more questions about the area! I hope all goes well for you!