The route of the disused railway line across the harbour and into Folkestone Harbour station has recently opened as a promenade (which they call the "Folkestone Arm"). It makes for a pleasant stroll.
Do visit the B-o-B Memorial at Capel-le Ferne if you can. It is a splendid memorial to brave young men. I note you won't have a car. There's a decent bus service (route 102) from Folkestone bus station with a bus stop outside the memorial site.
The same bus, but in the opposite direction, will take you to Hythe, one end of the Romney, Hythe & Dymchurch Railway. This is a gem which you should not miss. A round trip, across Romney Marsh to Dungeness takes a little under two hours but if you don't fancy that you can travel just to New Romney (35 minutes from Hythe). There is a museum there and a cafe which you can visit before heading back.
Lower Leas Coastal Park (which is a linear park along the seafront) is a delight on a fine day. (I've spent many a long hour at its entrance, monitoring the flows of pedestrian and cyclists passing through it, and interviewed loads of people visiting it. Everyone says how nice it is).
It would be well worth taking a trip by train into Canterbury (unless, of course, your coach company has included such a trip anyway, as many do)
https://www.canterbury.co.uk/
There's an hourly train service from Folkestone Central to Canterbury West, taking about 55 minutes, with a change at Ashford International. (You can also go from Folkestone West to Canterbury East, with a change at Dover Priory).
If you're on a coach trip there, my guess is that you're staying at the massive Grand Burstin Hotel (as it's the hotel that has entire fleets of coaches lined up outside it every morning). The locals love to run it down but I really liked it. (I'm not a great one for cooked breakfasts but I love the breakfast there!) Enjoy your stay!
I used to be involved with a "Paris Twinning" organisation. We did reciprocal trips. One trip we organised for our Parisian friends involved them travelling by coach and ferry from Boulogne to Folkestone (in the days when that town had a regular ferry service to France). They were billeted at the Grand Burstin (so as to be able to visit Canterbury, etc.). As part of the package we negotiated with the hotel, guests were treated to a day trip to Boulogne! Needless to say our guests did not take up the offer!