When researching options for our port stop in Montevideo, I randomly found a horseback riding adventure on Airbnb under the "experiences" tab.
The cost was only $20 for a 1 1/2 hour ride.
I highly recommend this ride. The farm is out in the country and about a 20 minute drive from the port. It was very easy to get an uber. The horses are very well trained and gentle. We rode through a forest and on to the beach.
We did a 10am ride. Had plenty of time to get back to the port, lunch on the ship, & then explore old town.