Despite being the island of magic, Florianópolis is infamously difficult to get around. I’m going to say this straight out: the best way to get around Floripa is with a motorcycle. The reason is that the major problems in Floripa transportation are terrible beach traffic, few roads, and infrequent/indirect buses; with a motorcycle you get around all those problems.
Car: Most Floripa guides would recommend you to get a car. Having mobility in Floripa is priceless as buses will not take you everywhere you want to go and will likely take hours. In Canasvieiras and Lagoa de Conceição, you can get a car for as cheap as R80/day ($32USD), or even less if you rent more days. The problem is that everyone drives, and there are very few roads on the island (I.e. only one highway to the north of the island and one local road), so in the summer, rush hour, and nice days on the weekends the traffic will keep you there for literally hours. The was a time that I was on a bus to Lagoa de Conceição, ran into traffic, got off to walk from Praia Mole (making a couple stops on the way) and still beat the bus two hours later.
Motorcycle: If you rent a motorcycle, you will feel like the king of the world as you zoom past all the traffic. Bikes can be rented for R60($24/day), at Lagoa de Conceição.
Bus: There are transfer terminals all around the island and if you want to go far, you generally have to make at least one transfer. For example, from Canasvieiras to the Center, I have to take a bus to the Canasvieiras Terminal (TICAN), then another to the Center (TICEN). It is an efficient system but the problem is that many lines don’t have that many departures (I.e. more than half an hour for each departure, generally more), so you end up wasting time waiting for the bus. The pro is that it is 95% of the time on time and you make transfers for free so the whole trip is only R2.90. It is worth it to consult the bus schedule and plan ahead. If you have an Android device, you are lucky because of this amazing App! Bus Maps Floripa was created by a UFSC student and has all the bus schedules that you can access even without internet, and all the routes in map format.
Hitchhiking: Not a bad choice for frugal, hitchhiking is quite popular in Floripa given its good safety and poor transportation options. If you’re a girl, expect to find a ride on a busy road on the beaches in less than 2 minutes. Floripa is a good place to pop your hitchhiking cherry =p Recommended only for the daytime.
Biking: I’ve written an article about biking. Basically, Floripa is a large island, has no bike lanes, and lots of hills, so biking is good for short distances/biking on the beach, but an impractical way to explore the whole island.