The most obvious day trip is to Lake Baikal, whose southern shore is about two hours' drive northwest of Ulan-Ude. Several small lakeside villages — Gremyachinsk, Goryachinsk — make a good base for a swim in summer (the water is heart-stoppingly cold even then), a hike, or a smoke-house omul lunch.
Ivolginsky Datsan is a day trip in its own right — see the things-to-do section.
Tarbagatay and the other Old Believer villages south of the city offer a glimpse of an unchanged 18th-century Russian religious community that has preserved its painted houses, costumes, and a cappella singing for three centuries.
Kyakhta, the historic tea-trade town on the Mongolian border (about 4 hours south), has a fine merchant quarter and a remarkable little museum — for those with extra time.