There is plenty to explore on your own. If you book through Rainbow Ranch Lodge you may call and speak to their concierge at any time to help arrange travel or activities. I will add in things to do below here, as I think of them.
Perfect place to go after hopping off the flight. Their quiche is the best and it's right on your way from airport to venue.
Yellowstone National Park
Yellowstone National Park,
May is a great time to visit the park. Animals sightings are guaranteed: the bison, moose, and elk are calving. Grizzly and black bears will be out and about. The park entrance is around 2 hours away from the Lodge.
Remember certain roads are closed during the winter season. If you are mapping out your travel and can't find roads through the park, they will be open in May.
Ousel Falls Park Trail
Gallatin Gateway, MT 59730
This is a very easy hike close to the venue, takes around 45 minutes. You could easily complete this in the morning before the wedding! Lava Lake is another trail nearby with a gorgeous view at the end, that will take around 4 hours.