A quick search on Kayak shows that there are some great prices on flights right now. Depending on where you're coming from flights are starting at $97 round trip from Boston, MA. Sites like Kayak and even the airline sites themselves typically offer an alert service that will notify you via email when flights go on sale. This can be an easy way to get a great deal on your next trip to Nantucket.
Expanded Ferry schedules Coming
Starting on May 18th the Hy-line Cruises fast ferry schedule offer expanded service times.
Starting on June 11th The Steamship Authority offers their expanded service times.
Always check with the ferry company for updated schedules or last minute cancellations.