Hi Paola
Sorry I can't offer to go with you, but I went myself as recently as 2 weeks ago so hopefully can give you the heads up.
I'm always wary of the 'stay here and pay the fine' argument for 2 reasons:
1. What if there's a massive overnight hike in the fine? Payable in USD? It's more than possible from this government.
2. Colonia is a wonderful, relaxing day out. Personally I'd rather pay more and get something out of it than just push 300 pesos down a hole.
Anyway, there's really no need to dread going to Colonia (unless, of course, you have special reasons for that). I'd suggest buying your ticket before the day of travel, that way you're at least sure of what time you're sailing, and you know the layout of the terminal etc. Once you're in Colonia, you don't really need to change your Arg pesos to Uru ones, as most (emphasis on 'most') restaurants and bars will accept your Arg money at 3 Uru to 1 Arg. Just ask before you order anything. Besides, there's a change office that you can't miss if you're walking into the historic part (just follow everybody else!) and again that's 3 to 1. On the same road there's a very helpful tourist office, who'll give you a free map and lots of advice.
Colonia is, quite simply, beautiful and well worth a day out in its own right. It's relaxing and the perfect antidote to BsAs life. Everybody's a tourist, so you can freely walk around with your map and camera. In fact, you can yell out 'Yo soy turista y tengo mucho dinero' and probably nobody would take any notice (that's tongue in cheek btw ;-).
I was there for ten hours (albeit with a friend) and didn't get bored once. If I'd been alone though, I'd still have been happy with 5/6 hours. Take a book, and enjoy the day.
I hope this helps. If you have any particular concerns, feel free to PM me. I'll be glad to help.
Best
Gary
If anyone needs leave or wants to take a one trip, let me know!! male or female I am a 33 y/o female from Miami. 
