If you are a nature lover or just like the peacefulness of countryside without the hussle and bussle of big cities and many people, this is the place for you. The animal life in the wetlands ranges from plentiful cayman crocodiles, capybaras, swamp deer to many, many, many species of birds. It is just a paradise at your fingertips out there. The many posadas or hospodajes (your accommodation options - about 20 of them) arrange for day or night boat trips to the lake and its marshes, horseback riding and trekking trips and probably other ones as well. So, once you book your accommodation and transport you can sort everything else once you get there.
There are luxurious posadas and also the most rudimentary hospodajes in Colonia Carlos Pellegrini, so you have plenty to choose from. We stayed in a mid level price range posada called Ypa Sapukai where the owner Claudia, speakes fluent English, so booking and arranging transport is not a problem. You can choose from half board (breakfest + dinner) or full board options to begin with. Beware though, there is not much cell phone coverage, so you either might get lucky and your phone works or it doesn´t (like ours). Well, if you don´t have a tri band phone, it will not work anyway :). We did manage to find an internet place as Ypa Sapukai´s was broken at that time, at Hugo Boccalantro´s house (just go to the tourist office and ask for a map, his house is on it).
Now about how to get to this place. To and from Posadas there is supposedly one sort of transport that goes on Tuesdays and Wednesdays, but you definitely have to have the hotel in Colonia Carlos Pellegrini book it for you (I was told it was 90 pesos per person). The other option is to take a private transport in 4x4 vehicle which is around 550 pesos per car. We unfortunately had to opt in for that one which put a hole in our pockets, however, was very fast (2.75hrs) and the driver waited for us at the bus station when our bus arrived. I asked him what was the longest time he drove from Posadas to Pellegrini and he implied that in a heavy rain it took him once 7 hours (ouch).
From Mercedes there is a bus in the afternoon around 2pm, I think . To Mercedes there is a bus in the morning between 3:30-4am and in the afternoon between 4-5pm (about 25 pesos per person). The trip takes between 3-4 hours, but can be longer or might not be happening at all if the rain is heavy. In that case, you might want to arrange for a private 4wd vehicle for 300 pesos with your accommodation manager.
Bus from Buenos Aires gets into Mercedes at about 6am and on the way back it leaves back for Buenos Aires at 9pm and 11pm I think. The companies to check for times are Fletcha bus, San Cristobal and San Jose. Trip costs between 85-100 pesos depending on the luxury.
Well now, what are you waiting for. You don´t have an excuse not to go now:).