Laguna Cejar is definitely a must-do if visiting San Pedro de Atacama, simply for the novelty of bobbing around in the extremely salty water. The water is a little chilly, but refreshing. Unless you enjoy hanging out the rest of the day with salt crusted all over your body, bring fresh water and a towel to rinse off with. The salt content is no joke. There aren't areas to change or rinse off, so be prepared. Paying for a tour isn't necessary, as other reviewers have noted, it's a short car ride and close enough to reach on a bike and the entry fee is only $2.000 pesos.