All great suggestions. Although a 50+ Bootcamp may attract a bit more homogeneous group of participants than simply a 50+ exercise class, for me it’s important to realize that there is likely to be significant differences in levels of fitness and capability among your participants. Also, emphasize lower intensity, more reps in strength stations on the circuits you design. I agree that hikes are a great choice for both fitness challenge and socialization.
Good luck, have fun.