All the guides in Drake Bay are part of an association and the hotels that offer packages go through these guides, so that’s why the types of tours and prices are all very similar. There are lots of really good guides there but two who we really like are Javier and Vincent. They’re both from Drake Bay and are super knowledgeable about the local plants and wildlife (and they speak perfect English). We gave links to their contact info in this forum question: https://www.twoweeksincostarica.com/question/store-luggagemore-corcovado/. If you’re just planning to do day trips, you can definitely wait to arrange tours until you get there. That’s probably the easiest unless you go through a hotel because it can be hard to get in touch with the individual guides. The only tours you really need to book in advance in Drake Bay are overnights at Sirena Ranger Station in Corcovado and the Tracie the Bug Lady Night Tour because those fill up fast.
Both Javier and Vincent do tours of Corcovado (highly recommended) and Vincent does the snorkel tour to Cano Island (or at least he used to a few years ago), which you should definitely check out if you want to snorkel because conditions are really good there.
Answer for Tours/hiring a guide for Corcovado