If I had to pick only one it would be Mayfield.

But if I was going to devote a day to excursions, I would do a long day and hit:
Black River
Pelican
YS Falls
Appleton
You could hit at least 3 of those 4 in a day (maybe all 4 if you dont stay long at each). I did the black river/ys falls/appleton; picked up at 9am back just after sunset.

The best way I have found to lower cost is to set something up with a driver now and establish a price. Then ask how much the price would drop per person if you added people. Then you can try and recruit people to join you (try asking here or on tripadvisor or other guests once you arrive).