MexiGo Tours has a good reputation for its trips that cover a range of cultural and nature activities in one day. You might be swimming in a cenote in one hour and sampling tequila the next. Not to mention visits to ruins and action further afield, such as boat tours to see flamingos at Río Lagartos. It can also arrange airport pickup in Mérida or Cancún.
Also offers bike hire per hour/day from M$25/150.