I went with the ATS Ruta Puuc bus that leaves from the Merida second class buss terminal at 8am on Sunday (169 pesos). That bus is great because it allows you to go to many sites but (1) it's quite rushed ie 30-35 minutes per site and (2) its last stop before returning to Merida is Uxmal, which means that if the bus is running late, you will hardly have any time to see Uxmal.
But there is a good news: you can basically forfeit your ATS return leg and stay longer: a AutoSur bus (white and yellow) picks up on the main road just outside the Uxmal site a bit before 6pm. You are better off getting your ticket in the morning to make sure you aren't in the middle of nowhere with no ride.
Uxmal is beautiful and definitely warrants getting a guide so that you can learn all you need to know and benefit from a better understanding of the ruins. You can also climb to the top of the acropolis and just enjoy the views
Carry water because as usual it does get warm, but don't worry there are options near the ticket counter
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