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Volcán Parinacota
Photo: Gerd Breitenbach, CC BY-SA 3.0 via Wikimedia Commons
A day hike with guide that starts at 01:30 at night, so you reach the top right before sunset. Transport via 4WD is necessary and provided with a tour. There is actually trails from both the Chilean and Bolivian side, so you might meet some people heading up from the Chilean side to the top of this peak separated by the border.
Parinacota, Parina Quta or Parinaquta is a dormant stratovolcano on the border of Bolivia and Chile. Together with Pomerape it forms the Nevados de Payachata volcanic chain. Part of the Central Volcanic Zone of the Andes, its summit reaches an elevation of 6,380 metres (20,930 ft) above sea level. The symmetrical cone is capped by a summit crater with widths of 1 kilometre (0.62 mi) or 1,000 metres (3,300 ft). Farther down on the southern slopes lie three parasitic centres known as the Ajata cones. These cones have generated lava flows. The volcano overlies a platform formed by lava domes and andesitic lava flows.
© OpenStreetMap contributors
Volcán Pomerape
Even though lower, this one is more difficult than Parinacota.
Laguna Sora Pata offers this rare and protected species of ducks with their surprisingly large nests on the lagoon. Quite picturesque with the snow covered large mountain in the back. If you follow the trek further northeast, shortly after the end of the lagoon you can regularly see large amounts of viscachas.
The trail takes 8-12 hr, but add 1-2 hr for the geysers, hot pools and local fauna. Highlights are the many animals along the way, especially near and around the three lagoons: Casiri Macho, Sora Pata and Chiar Kkota. Many people do it in 1 day, but 2 days is possible if you have a proper tent and warm sleeping bag. There are marked camp sites along the trail, but generally many opportunities exist. The trail can easily be done without guide, but it is better go in a group if you are not a regular hiker. The trail actually enters Chilean territory, but only for about 3-5 km, after which you will return to Bolivia. If you stay on the trail and do not enter further into Chile, this is not an issue. The trail is well visible most of the time, but you should bring GPS and a proper map to take the right turns and trails.
Surprisingly versatile with hot pools, geysers and mud pools. It also has some little and free and natural pools you can soak in—just select a place in the river that has reasonably much space and is hot enough. But also, check out you map app to find them. The very first pool is already great for a dip. (Control the heat of pool with rock near small waterfall, which blocks the water to the geyser. You can help making the pool bigger & deeper, namely where the water is calm and the hot geysers are. Maybe even bring a shovel, but do not tell the locals about this secret place, or it will be gone soon. Allow for enough cool water to flow by outside of the pool. And watch out, the soil can be very hot.)
In case you are taking your time with the Tres Lagunas Trail and camp overnight, you might in addition consider this interesting and exciting side-tour with magnificent views of the Lagunas, into the Sajama valley as well as Nevado Sajama itself. Allow for an extra day and night to conquer this mountain, but camping near the Laguna Chiar Kkota is very picturesque and convenient at the same time. In winter, the trail can be impassible.
Quite expensive, considering the hot spring along the Salar de Uyuni tour is just Bs. 6. There are other (private) hot springs nearby, try this hot spring or this one. From the latter a trail north leads to two more hot springs, which might be for free.