I tend to be a polo-type guy because I think they breath better than t-shirts.

I bought a bunch of golf polos at Kohls for our last trip (Vietnam and Cambodia) and they worked great. Grand Slam Air Flow are probably my favorite because of the way they fit me and the styles, but they are all pretty similar. I have a couple Izod, and maybe one other brand.

For rafting I usually just wear a generic dri-fit type t-shirt or a swim shirt. You can get all sorts of cheap breathable exercise/running shirts at Walmart for under $15.

I would probably wear the t-shirt style for ziplining and repelling too because the collar of a polo might be annoying with the gear you have attached, but I usually wear polos for hiking.