We were in Montevideo last year as a port stop on the Royal Princess. We did not do an excursion, but took a "free ride" to a sweater and souvenir shop (lots of those on the pier) and then just walked through the town, stopped at all of the squares and plazas, had lunch at a market near the pier. It was a nice leisurely day.
One of the best things about Montevideo is that as you get off your ship, look to the right (you will have your back to the ship) and you will see a little building that says something about telephone and internet. This is a facility which is their for tourists' use. You can check your email for FREE for 15 minutes (or longer if they are not busy). You can also get lots of tourist information. The best thing to do if you are not on a tour is to go there as soon as you get off the ship. It usually gets busy in the afternoon.
Have fun on your cruise.