Can you do a day trip to Lake Titicaca from La Paz?

Yes, doing a day trip from La Paz to Lake Titicaca is possible, but it turns into this long, exhausting stretch of travel time. Most tours leave way early, and it takes about four hours to get over to Copacabana each way.

In practice, the usual plan is a quick lunch in Copacabana, then a short boat ride toward the southern side of Isla del Sol, and then you head back fast to La Paz before nightfall. It is doable, but I highly recommend staying at least one night in Copacabana so you can actually appreciate the views, instead of spending something like eight hours inside a bus.