The Salar hogs the glamour, but the area nearer La Paz would immensely reward spirited exploration. With a clever itinerary, one could travel in a truly fast and light backpacking style on a dual suspension rig. On an imaginary future trip I imagine riding down Yunga Cruz to the jungle and then up The Death Road.
Or bring an unstoppable Fat Bike for a more deliberate, deep backcountry effort.
Here are some maps for riding and walking in the Yungas and mountains north and northeast of La Paz, including the Death Road. All of them are available in La Paz; I post them here for pre-trip planning.