Hi Liz - sorry its taken me a while to get back to you - just returned from holiday in Lake Garda!
Ajjaccio is definately easy to do on foot from a cruise ship - have done so a couple of times myself. Menorca ditto - Mahon is very pretty and easy to do as well -apart from a bit of a climb up some steps to get into the town centre.
We have called into Palermo before, which was ok - ish, but otherwise I don't remember the ports we called into in Sardinia and Sicily as being that good - however, the ships tend to vary the ports of call on these islands, so I can't speak for all of them. Did a lovely ships tour to Taormina in Sicily, which I would recommend. Have a look at the threads on the cruise discussion forums on this site, if you haven't already done so.
Which ship are you going on? We love cruising and have just booked a med cruise for next summer, on the Thomson Spirit.
Get back to me if you have any more questions.
Gill