Ditto on the merino socks for riding. I've tried a lot of sox and underwear combos, and the SmartWool Merino sock liners are great. They are a bit thinner than regular socks but noticeably cooler in Texas weather and MUCH less smelly after use.
I've used the nylon/spandex type shorts for underwear when riding and they've worked really well at staying comfortable far longer than other types. Look for brands that don't have seams across the butt area. Pack small and wash/dry easy.
Recently I've tried the Ex Officio's and they are really comfortable. I think I'll carry a couple pairs of those and couple pairs of the spandex on long trips.
Nothing like chatting about underwear amongst friends