Fit walkers can hike the crater rim trail in about six hours; another path leads down to the water on a zigzagging trail. At the bottom, you can head out in a canoe or kayak (per 30 minutes $3). The hike down takes about half an hour and more than twice that to return (you can ride a mule back up for $20). The alkaline lake water is not potable.
Guides for hikes further afield cost about $35 per day; inquire at the information center near the crater edge. One excellent option is the hike past the laguna to the nearby village of Chugchilán.
The information center also rents bikes and helmets (per day $40).