Be sure to take something wide. A lot of the thermal pools look great from low and wide, especially Grand Prismatic.

Also, a good CPL to cut reflections. Mammoth Hot Springs for example will look absolutely surreal through a polarized filter, as will the above pools.

I think the 500 will be amazing. If you bring it, then the 70-200 seems appropriate for landscapes and larger wildlife. I also think a second body, even if just your M, will save you some time and hassle if bringing the 500.

Check out Dragon's Mouth for a completely nonvisual thrill. Underrated attraction in my opinion.