Akumal is an awesome little place and makes for a great day trip, but it in no way compares with Isla Mujeres. If it’s a choice of one or the other, Isla wins, hands down!
Take a colectivo to Cancun for 3 bucks. Get off at Cancun bus station and grab a taxi to Gran Puerto Cancun ferry docks (not much $$). Take an Ultramar ferry to Isla Mujeres - round trip 8 bucks, 20 minute ride. Try to grab a seat up top in the open air - the views of Cancun and the water are beautiful.
Here’s a link to the ferry schedule: http://www.granpuerto.com.mx/isla_mujeres.htm
Once you arrive, rent a golf cart (about 55 bucks for the day) - there are plenty of rental guys buzzing about near the ferry dock so you won’t have trouble finding a place to rent from. Take a tour of the island (I reccommend a Can-Do map of Isla to make the most of your day - see http://www.cancunmap.com/). Try a stop at Playa Lancheros for lunch - they serve an amaaaaaazing Mayan dish called TicnXic…absolute heaven If you’re looking to snorkel, try Garrafon Park, or go out with one of the local guides.
Once you’ve finished with your island tour, do a little shopping in Centro and then laze the rest of your day away on Playa Norte, a beeeeeautiful beach on the North end of the island. Sergio’s Playa Sol, a cool little beach bar, makes the perfect spot for catching the sunset before hopping a ferry back to the mainland.
You can’t beat Isla Mujeres…go and see for yourself!
Here’s the last sunset I caught on Isla…
Happy travels, wherever you end up