Easily reached from Zeebrugge/Blankenberge and train fare is cheap. Eur3.80 for weekend fare, around Eur5.00 on other days. No need for ship excursion. Brugge is small so manageable on foot. Food tend to be expensive, around Eur22.00 for a pot of mussels. Lots of chocolate shops with all different pricing, although the products looked suspiciously similar...
Port reached by tender boats. £1 bus fare for 90 minute round island trip, bus stop for 7A bus just across the road from the port area. St Peter Port very small with surprisingly few souvenir shops. We were in Guernsey on another cruise about 5 years ago and much has changed, not necessarily for the better. The old market area has been converted and is now occupied by the same chain stores you find on any British High Street. Many seafood restaurants. We chose to eat at Le Nautique with great views of the harbour and good food. 2 course set menu for £15.50 is a very good deal considering the quality of the food.