At the airport there are car rental companies. Please note, they charge a HUGE deposit of $1500 that they hold for over a week. Don't bother to get GPS if you plan ahead and get the international rate for your cell phone plan (typically $10 extra for 1 month). It's about a 45 minute drive with no traffic to Sayulita. Once you get out of the city and begin your uphill climb towards Sayulita, there are 2 roadside stands with coconuts and all kinds of great snacks; they are literally on the side of the road.
There is a Walmart on the way out of town and a 'SUPER MERCADO' grocery store. You can go in and get all the water you want, as well as beer, ice, ice chest, limes, etc. All the essentials will be in the supermarket. There is also a small market in town that sells bottles and gallons of drinkable water.
https://www.rome2rio.com/map/Puerto-Vallarta/Sayulita-Nayarit-Mexico The link provided above will give you details on buses that go to Sayulita. It is very frequent. There is a bus stop at the Wal-Mart so you could potentially kill two birds with one stone, get your groceries/water, and catch a bus ride to Sayulita. You can also get a taxi or an UBER. The cost from the airport by taxi is approximately $50, with the return cost being approximately $60.
Yes, there are HOSTELS and a campground, that I believe has hammocks (CAMPGROUND: El Palmar del Camarón)
I have found R/T flights out of San Francisco for as low as $325. San Francisco and Southwest from Oakland do have direct flights, everywhere else has layovers either in LAX or Mexico City.