A superb half-day hike from Theth is to the Blue Eye, a natural pool of turquoise waters fed by a small waterfall, up in the mountains to the southwest of Theth. The walk will take you through forests and steeply up into the mountains; in summer it can get very hot, so you'll probably be keen for a swim when you get to the pool. But, be warned: the water is glacier cold. Are you brave enough?
It's a popular walk, so a guide isn't really needed, but do get detailed instructions from your guesthouse. Drinks and snacks are available from near the pool but it's sensible to take a packed lunch with you.