The first thing that popped onto the radar when I started to look into Sayulita is the Marietas Islands. It is a ecological reserve off the coast which has a interesting beach/cave (essentially the cave collapsed, leaving a crater like beach). I put it on my list of things to do (among the millions of other people who visit this area). Upon arriving - we heard a variety of reports from the hostel guests on experiences and what they paid.
I started to get the impression the islands had become a bit of a tourist trap. Regardless - my girlfriend really wanted to go, so she went out hunting for the best deal. She seemed to find it with the Chica Locca trimaran. Whale watching, open bar, island tour, kayaking, stand-up paddle boarding, snorkeling, water slides (on the boat) all for $85 - for a trip that is around 8 hours (I think) from the parking lot in Sayulita - not bad for $10/hr.
Frankly, I could've done without the whale watching - which mostly consists of harassing humpback whales trying to feed. Maybe that's just the hippy in me - that said - we did get to see a lot of whales...which was cool.
The staff was very friendly and the banter was funny (a canned tour - but still fun). The drinks came fast and furious. I generally limit my booze intake on the water - I feel it's a bit of a recipe for disaster - but I needed a bit in me to dull my overwhelming fear of sharks (my kryptonite). The pina colada's went down easily (I expected watered down drinks which wasn't the case - they didn't skimp on the booze).
I think I liked the kayaking the best - bombing around in them with my girlfriend was a lot of fun.
The snorkeling was good - a first for me. I got to put my tough camera to the test (overall I took 300 photos - 3 of which are decent).
The island tour itself was quick but unless the beach was deserted I wouldn't want to stay there much longer than we did. We explored a couple of the smaller caves which was a nice mini-adventure...a safe way to feel like Indiana Jones.
Overall it we both felt it was well worth it and were pleasantly surprised (I went into it with no expectations - maybe that is the trick).