Cobán [1] has a variety of nearby attractions that can be visited in one day as well as numerous outfitters that can get you there. Popular day trips include the pools at Semuc Champey [2] and the Lanquín [3]. Although these can certainly be done in a day, they are worth a stay of at least one night, as there are comfortable accommodations and plenty to see and do. If you have the time, stay overnight in Lanquín.
Other day-trip options include the Biotopo Mario Dary Rivera [4] and Chixim Church [5], both south of Cobán.
North of town, along Km. 24 on the road heading to Chisec [6], is the Sachichaj Waterfall [7], another increasingly popular attraction. Most of the following tour operators can get you there.
Links:
[1] http://www.moon.com/destinations/guatemala/las-verapaces/coban
[2] http://www.moon.com/destinations/guatemala/las-verapaces/lanquin-and-semuc-champey/semuc-champey-natural-monument
[3] http://www.moon.com/destinations/guatemala/las-verapaces/lanquin-and-semuc-champey/lanquin
[4] http://www.moon.com/destinations/guatemala/las-verapaces/cloud-forest-biological-corridor/biotopo-mario-dary-rivera
[5] http://www.moon.com/destinations/guatemala/las-verapaces/coban/sights/chixim-church
[6] http://www.moon.com/destinations/guatemala/las-verapaces/northern-alta-verapaz/chisec
[7] http://www.moon.com/destinations/guatemala/las-verapaces/coban/near-coban/sachichaj-waterfall