There are three options:
1) take Bell trail and go to 'The Crack' and back(7 mi RT),
2) continue up on Bell trail past Bell Crossing to the mesa and return (~10 mi RT),
3) follow Long Canyon trail from the trailhead returning on Bell trail (~17 mi RT).
I pursue option 3, but if somebody commits to easier option, I'll downgrade.
I believe that Wet Beaver pools still hold water, so swimming in them will be an integral part of the deal
I would also be interested in changing this to Fossil Creek, but I suspect that the area will be packed and I would need a ride in from the pavement to reach the trailhead at fossil.
I'd stick to Wet Beaver, because it can be less crowded, it's not a 'canyon' hike as Fossil Springs (i.e. you're going back downhill, not uphill), and I've never been there before.
I was at Fossil 2 years ago. It was actually very nice, and I would like to see the area post deconstruction. Depending on your transport, you can take the Flume Trail instead of the Fossil Creek Trail. I did the Fossil Creek Trail on a day where it was about 100 and I got down there around Noon. I hiked out in the high 90s around 7pm.
If I do Bell, I prefer the shortest route so as to skip the long hot hiking. I've done the Mesa two times, maybe three, and it isn't that much fun after the first time. I would not want do the 17 mile loop on a day when we are forecast to have 100s in the valleys. I think I would need a medivac around mile 10.
If we want to be in the water during the very heat (12n - 4pm), then it'd make sense to start from parking lot at about 10am and do it just to the Crack and back. What do you think?