This is kind of hard for me to answer with one pick.
I have a Canon 60D. Not the lightest for backpacking but for solo picture taking, I like being able to use the remote and having a screen that flips so I'm able to view what the camera sees facing the camera. I can set up the shot without having to rush or keep trying repeatedly to get the right shot. With the right lens it takes great pictures.
For lightweight, and being able to use on the spotting scope, I've been looking at getting a -Canon*Elph 100HS (shoots*720p or 1080p HD video) 12*MPX*photos*compact HD digital camera or something similar.
I also like a Kodak point and shoot. I have a EasyShare Z981, 26mm-676mm equivalent that I got year or so back, It takes great pictures but it is not super light but it is not super heavy either. They have models that reach out to 840mm equivalent now and have better processors in them and shoot video in 1080p. For the money these cameras I think are a good buy for someone that wants to shoot subjects at all sorts of distances without a whole lot of weight to carry.