The airport is the most convenient place to exchange money, and the rate there is pretty much the same as elsewhere in Cuba. It's quite easy to find a taxi at the Havana airport which will take you directly to Vinales, a drive of 180 km or about 2.5 hours. Not sure what the price is, but probably a little less than the daily rate for car hire + insurance + gas.
About halfway between the airport and Vinales is the Las Terrazas Biosphere Reserve. It is a region reclaimed from the devastation of 19th Century coffee farms followed by severe erosion and clear-cutting, now a pleasant wooded area. In the village there is an organic garden which supplies the town and single hotel. Assuming you have time on the day of your arrival to stop there, you might negotiate with your taxi driver to do that.
If you don't have time for that, just outside Vinales there's Finca Agroecologica El Paraiso, which offers excellent lunches and guided tours around the finca (farm). It's more set up for an educational tour than the village farm in Las Terrazas, where you might not be able to find anyone who speaks English. Once back in Havana, I agree with Rob that the Alamar organiponico is worth visiting. It's about 10 km east of Havana, easily reachable by taxi.
Cayo Megano, also known as Cayo Parisio, is a good choice geographically, but I'm not sure how good the snorklling is, as what I recall of it from one visit (quite a long time ago) it has a lovely sand beach and clear water. But maybe there's a reef nearby that I didn't notice, as I did not snorkle on that trip. There is an open-air cafe on the beach, not much else. You would make arrangements to get there at the Cubanacan office in Vinales. They run a daily bus--8 AM departure, I think-- to La Palma where the ferry departs for Cayo Juitas, and also where you take the little boat (arranged by the Cubanacan office) to take you to Cayo Megano. You could spend a day there, then, when you get back to La Palma, rather than return to Vinales, get a taxi to take you on to Havana--about a two-hour trip.
One other possibility: there is a casa about 500 meters from the La Palma ferry dock which is on a privately-run farm. You pass right by it as you go in. When I stayed there, the owner took me around the farm and also arranged for me to explore the big state-owned farm next door. I don't recall if either was organic, although most agriculture in Cuba is for economic reasons--if not 100% organic, at least mostly. (I was told a few years back that it only raises five crops for export that meet the standards of "organic" in Europe.)
Another possibility would be to choose a place to stay in Vinales which is a farm, like Casa Cafetel, which I believe is on a coffee farm. Then there's
