Here is how I ended up doing Uruguay in-a-day.
From Buenos Aires there is a one hour ferry ride across Rio de La Plata to the town of Colonia. Immigrations is even handled right at the ferry terminal. Once off the boat in Colonia, it is a ten minute walk down to the old city (barrio historico) where one can stroll through the cobblestone streets, visit the numerous museums on a day pass, climb the lighthouse and head over to the waterfront pier and watch sailboats coming in to dock. A few minutes further east down the road and there are plenty of restaurants (read: ice cream shops) to enjoy. A half is day is plenty to explore this portion of town, but if you end up getting the late ferry like me there are always the sandy beaches nearby to relax on, cafes for afternoon coffee and alfajores or postcards to send!
4 passport stamps and 14 hours later, I was back at my hostel in Buenos Aires! Hopefully my next visit to Uruguay will include Montevideo and be at least an overnighter!