Mesa Fall's is one of the most scenic waterfalls in the west. It separates the upper and lower Henry's Fork in dramatic fashion. Upper Mesa falls has a fee but is well worth it to walk the board walk right up to the edge of the falls. Lower Mesa falls is free and a quick pullout from the main highway. Both are worth your time. Don't forget to feed the fish at Warm River!
We got engaged in Yellowstone so we highly recommend visiting the park! Summer traffic can and most likely will be brutal. Go before 7am to beat the morning rush. When exiting the park in the evening you can expect long delays at Madison Junction. It's only 30 miles from Old Faithful back to West Yellowstone but this drive can take two hours or more in evening traffic.
If you're driving up past Rexburg, stop by June's Place for a coffee and treat! Rexburg is where Dan grew up!