It will be warm. Mid-80's, up to 90 degrees during the day and probably around 65-75 during the early mornings and nights. There is a nice breeze usually coming in off the water. Our wedding day will be right on the beach so the breeze will be even better.
Sayulita is a very small town. No matter where you stay - you'll never be more than a 10-15 minute walk to anywhere else in town.
You are going to want to carry and pay for most things with Pesos. It is the easiest option. Credit Cards are accepted at some places but others are Cash Only. You can convert cash to Pesos ahead of time if you'd like - but we find the easiest option is to just use the ATM's in town like you would anywhere else. The ATM fee is minimal and you don't have to worry about conversion fee's somewhere else.
There are multiple locations in town where you can rent a golf cart for $5-10 dollars a day. This option will help you get anywhere in town quickly and easily.
Stick with ONLY bottled water during your stay in Sayulita.