The Oban to Craignure ferry operates very frequently, so unless you have a specific sailing in mind, you could wait until you arrive in Scotland to make your reservation. If you do have a specific sailing in mind, you can go on the CalMac website and make your reservations at any time before you leave for your holiday.
There are many hikes and walks on Mull, ranging in difficulty from easy to strenuous. The best thing to do is go to the WalkHighlands website www.walkhighlands.co.uk Click on the map of Mull, then click on the area of Mull in which you are interested. There are good low level walks/hikes on Iona and Ulva. Both are reached by ferry.
There is only one "town" on Mull - Tobermory. The seafront is particularly attractive, with the houses and shop fronts painted in different pastel shades. Walk up the hill in to the residential district to get a sweeping view of the harbor.
The rest of the main populated areas on Mull are villages. Craignure, where you'll be landing; Salen; Pennyghael; Bunessan; and Fionnphort, from where the ferry leaves for Iona. The real beauty of Mull is that it isn't crowded, except for tour buses going from Craignure to Fionnphort. If you have time, I'd recommend taking the west coast roads, especially the B8073 from Tobermory to Gruline. It's slow going, but worth it. Plus you can visit Ulva from Oskamull.
Hope that gives you enough information.
p.s.: If you're looking for a castle to visit, Duart is a good one. It's just south of Craignure. You'll see it from the ferry as you sail in to Craignure.
p.p.s.: Ferry fares are 27.50 UKL round trip for your car, and 7.40 UKL round trip for each adult. Sorry, my ancient computer keyboard doesn't have pound signs. :(
p.p.p.s.: Depending on where you're going after Mull, you may want to consider one of the alternate ferry routes back to the mainland.