We are considering going to Lake Titicaca after a trek on the Inca Trial but it seems expensive and time-consuming to get there for a short visit.
We definitely want to go to Colca Canyon and Arequipa afterwards.
Is one full day at Lake Titicaca enough?
Or should we skip Lake Titicacca and allow extra time in Cuzco and Arequipa/Colca Canyon?
Is it doable? It is if you mean that you get in the night before, stay a full day and then another night and leave the next morning. Otherwise, not really. While it is physically possible, whether you should do it comes down to your tolerance for being time-crunched and just breezing through to see what there is to see.
The bus trip from Cusco to Puno takes about 5 hours, so you'd get into Puno with a little time to spare; you could use it to find out about a trip for the next day (alternately, you could book a pre-planned itinerary ahead of time). The boat trip out to the floating Uros Islands takes about 30 minutes one way, whereas the trips to Isla Taquile or Amantaní takes about 3 to 3.5 hours one way—it's easy enough to find a boat that'll take you to Uros and Taquile or Amantani. Be aware that if you go to Taquile and want to have a look around, you need to climb 500 steps to get to the top of the island, where everything is located. I'd wager if you want to do Uros plus one other island, you're looking at a 10-hour day. Then you'd get your night's sleep and hop back on the bus.
Personally, I would suggest you skip it. It breaks my heart to say it, because I loved Lake Titicaca, but we stayed there for three days and wished we'd had more time. I worry you'd just be setting yourself up for stress and disappointment—not in the location, but in being unable to stay longer.
I think you could easily tag another day onto Cusco, if only because if you get tired of what's in town, it's so easy to hop out to some nearby villages.
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