Gravensteen Castle - Highly recommend the audio tour
Belfry of Gent
St Bravos Abby
Taking the train from London, you can explore multiple cities in Belgium. Once in Gent, there is a public bus to take you from the train station to the central downtown area. Everything downtown is walkable as it is a small city.
Hotel Gravensteen is near the Castle, is very reasonably priced, and includes an absolutely wonderful breakfast.