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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.
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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.
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.
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.)
Volcán Pomerape
Even though lower, this one is more difficult than Parinacota.
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.
A little cheaper and free if the lady is not there. So, go early or late (coming from the Tres Lagunas Trail), or just be lucky (e.g. in off-season).