We are going for 4 nights in late February, followed by 4 nights at Hidden Beach.
My only advice so far would be to carefully consider how you will get to Chicquila, from which the ferry to the island departs. It's about 2.5 to 3 hours drive from Cancun. It costs about $200+ for private transfer. There is also a public bus, but the last bus may leave earlier than you would like on your day of departure and you have to transfer in downtown Cancun.
Since we are going on to Hidden Beach we decided to rent a car for the whole trip even though it'll sit unused at the ferry parking lot while we're in Holbox.
We're also hoping to stop by Playa Del Carmen on the way to Hidden Beach for lunch and a little shopping.