Getting to Isla Mujeres can be a bit tricky. There is an easy way and a more challenging/exciting way. You could always just fly into Cancun and take a taxi to Puerto Juarez, then catch the boat to the island. Or...you could hop on a bus from the airport into Cancun Central. The buses gather outside the airport, so it is easy to catch one. Make sure you have pesos though, because you'll pay in pesos before getting on. This bus ride is 30 minutes. Once at Cancun Central Station, head outside of the bus station to the main road. Start asking around for Puerto Juarez and soon people will guide you across the street where you will take the number 6 bus. The front of it should say Puerto Juarez along with a few other places. Once you catch the number 6 bus it takes about 15 minutes to get to the port. When the bus arrives at the port it will look like a hotel. You walk through a parking deck (there should be signs for the port) and will arrive at Puerto Juarez. When you buy a ticket for Isla Mujeres it is a round trip ticket, so be sure to store the other half in a safe place for your return. The boat takes about 30 minutes. Once on the island you can take a taxi to your hotel.
Most of the hotels are on the North Island, while the resorts are on the South Island. We like to stay in a hotel because it allows us to get out and see more of the island and culture. The hotel we like to stay at is called Hotel Cabanas Maria del Mar. ($75-$130/night depending on when you go.) It is located on the nicest beach (in my opinion) of the island. The hotel is pretty basic. It is composed of a few different buildings (or cabanas) with hotel rooms. Each room/cabana contains a basic bathroom, fan, air conditioner, tv, and small fridge. The rooms are not exciting or luxurious, but they all have a great porch for outside lounging. The hotel grounds consist of beautiful gardens and a pool. The beach is directly in front of the hotel. Next to the hotel (and on the beach) is a swing bar and restaurant. The hotel gives you a free breakfast voucher for the restaurant every morning which consists of coffee, orange juice, and toast. Aside from that you can order more, or go elsewhere for breakfast. For lunch, I recommend their fish tacos. The restaurant also has a treehouse platform where they have 3 yoga classes a day.
The primary mode of transportation on the island is golf cart. The island only takes about an hour and a half to get around, so renting a golf cart to explore is a must! We have always rented our golf cart at our hotel. You can have it for the day for about $50, and you will need a drivers license.
If you stay on the North Island, the main part of town will only be a few blocks away. We always enjoy going into "town" for dinner and ice cream. On the main strip you can find just about any type of restaurant and food. Asian, Italian, Mexican, seafood, vegetarian...etc. Most of the seating is on the street, so you can enjoy people watching and various street performers and musicians. In this same area there are also lots of little shops. You can buy souvenirs, bathing suits, hats, sandals, sarongs etc. At the very end of the street you will find the town square. Local street venders like to set up there, and it is always fun to try their foods. That is also where the grocery store is located. The grocery store is great for sunscreen, beach toys, beverages, and snacks. We usually stock our fridge with drinks and snacks at the beginning of the week. That way we can sit on our porch at the end of the day and have a cool drink before heading out to dinner.
Even though Isla Mujeres is a small island, there can be a lot to do if you look for it.
Rent a golf cart - already mentioned above
Beach bar hop - if you walk along the beach you can find all sorts of fun little beach bars
Snorkeling/diving - there's a variety of this around the island
Turtle Refuge - you can take a taxi or golf cart and check out all the turtles
Dolphin Discovery - First off, you have to pay for this. You can either pay to swim with the dolphins, or you can pay to enter the resort for the day. If you pay to enter the resort for the day it is about $30/per person, but that includes towels, all you can eat AND drink. Plus, you can walk out to where they are doing dolphin stuff and watch from the docks.
Ruins - at the very south end of the island there are a few small ruins and some fun trails to walk right along the cliff and water. ($5 entrance)
The Joint - this is a Rasta bar that you will no doubt drive past if you rent a golf cart. I HIGHLY recommend this place. They have the best/strongest drinks on the island and all of their food is great. Plus, they have a fun atmosphere.
Day Resorts - these are scattered around the island and each offer different things. Some have water slides, others have snorkeling and stand up paddle boarding options. Some are free entrance and you pay for food and drink, others are a fee but everything is included for the day.