We went to Yellowstone for the last time today. We didn't see much (just missed Great Fountain again), but we did bring bikes this time. It was great and I highly recommend them if you're going to Yellowstone. We were able to ride out to Lone Star Geyser. The boys were in a bike trailer, but older son is close to being able to go on his own. It's five miles round trip to Lone Star, so biking makes it much faster. It's a lovely geyser. It erupts every three hours and there's a logbook at the geyser so you can get an idea of when it will be erupting next. It's near a river and the boys had fun playing in it while we were waiting.
To get to Lone Star from Old Faithful, head towards West Thumb. Just past Keppler Cascades, you'll see the sign for Lone Star. Turn in and park, then take a lovely ride down an old road to Lone Star. It's considered backcountry, so there's not a lot of trails and no boardwalks. Make sure that if you do go to backcountry areas to pay attention because there are hot places that might not be noticeable and a thermal burn miles away from anything wouldn't be fun.
We also biked around the Upper Geyser Basin. That was great. The bike trail there is over a mile, so that also saves a lot of time. There are plenty of bikes racks available if you want to switch to walking on the boardwalks.
It was sad to walk around Geyser Hill this evening. At least our parents don't live too far from Yellowstone. We'll be back again.