I lived in Bolivia for 7 months. I really loved La Paz. It's right in the mountains, and you have access to Copacabana/Lake Titicaca, and Tiahuanaco.
La Paz is just a beautiful city, with nice cafes, shopping, art, history, huge markets and more. I would live there again in an instant. You would also have easy access to Peru. The weather in La Paz is usually cooler because of the altitude.
I spent most of my time in Santa Cruz. It's on the eastern side of the country, and is hot and dry. It doesn't have as much to offer as La Paz, but it provides fairly easy access to the Amboro rainforest.
Amboro has hiking, waterfalls and nice flora, but it is not developed- there are no paved or cleared trails (no trail markers, no lights), stairs, or anything like that. We hired a guide, who took us out for the weekend. You need to bring EVERYTHING you will need, because there are no facilities once you start hiking.
When I lived in Bolivia, I spent a week flying around the country. I flew from Santa Cruz to Tarija, to Cochabamba, to La Paz, to Sucre, then back to Santa Cruz. It was VERY easy and affordable. I got to see a ton!