It's beautiful, small, quiet. I love going there to renew my tourist visa, unfortunately I got harassed two times about the number of stamps in my passport so last time I went to the immigration office here in BA. If you go for the day you can spend the day strolling the streets in peace (where the cars even stop to let you cross). If you spend a weekend, you might get a little bored but if the weather is nice you can rent a moto cheaply and it's a blast!
Colonia is a great place to go for a quiet, relaxing day trip away from BA. There's not much to see other than the colonial town itself (which is beautiful), the river coast, and just walk or relax. We go there from time to time to have lunch, enjoy the afternoon, and come back the same day. If you want more than that, take your driver's license and rent a golf car (or scooter) to drive around. You can go visit the old bull-fighting stadium ruins and ride along the coble stone streets. It can be fun. Golf car rentals are right outside the ferry terminal.
Hey, like they said above, colonia is a place to go and relax. eat some chivito. go to the fair. walk along the beach. look at the pretty old buildings. rent a scooter. It's not Buenos Aires, but i wouldn't say that there is nothing to do. I've been there a few times now and I always have a good time- a nice breather from the big city. Also, if you want to see more of colonia, you could always try a tour. they would be able to bring you to places that the rest of us would never know about!
Its a nice quiet place, quiet expensive, but nothing so special if you visited another colonial towns in South America. what I found interesting is to cross the Rio de la Plata...anyway is always nice to change the city for a quiet back on time place for a couple of days...enjoy it!