Eat a meal at Kiwi's, pick up a pair of bison wool gloves at the Yellowstone Trader as a birthday gift to yourself while in West Yellowstone.
A nice 1/2 day hike in YNP is on the Howard Eaton Trail from just south of the Mammoth Restroom (in Mammoth Hot Springs), up behind the terraces, up through the Hoodoos, loop north and back down on the Snow Pass Trail and back down to your car. Pikas, marmots, elk, bison, bears, ravens, mountain bluebirds, wolves nearby, steaming clouds smelling of sulpher, boiling water, wildflowers, bones, nice views... it's all there.
Gotta stop into Vargo's Books and Jazz store in Bozeman, an unparalleled selection of both!
The local brews in Red Lodge MT are outstanding.
Happy Birthday and keep your eyes on the road.