I can thoroughly recommend the round robin trip in South Devon, which has a variety of start points, and can include sea trips or river cruises.
We started from Paignton at about 10am and boarded the Dartmouth Steam Railway, pulled by the steam engine Hercules for a glorious trip along the coast south to Kingsbridge, and the train ends up running along the estuary of the River Dart.
We went over on the passenger ferry to Dartmouth and had about an hour exploring the Church, which is well worth a visit, and the market, but did not have enough time to walk out to Dartmouth Castle, before the the river trip around lunch time.
A river trip on Cardiff Castle vessel up the River Dart to Totnes was next, passing Greenways (the home of Agatha Christie and now in National Trust hands, with a lovely garden down to the estuary – and can be visited from a stop on the Dartmouth Steam Railway). The sun was shining so we sat out on deck and looked at cormorants, swans and other birds as we cruised up river.
There was plenty of time at Totnes in the afternoon, and we visited the Guildhall and the Elizabethan House Museum, both very interesting before having some tea, and catching the bus back to Paignton from the harbour at Totnes (the bus is included in the package and you get a good view over Torbay as you come down to Paignton).
You can do the round robin the other way around, or begin in Torquay, but it is a fun day out with plenty of options, and the timetables and fares can be found on the Internet. An excellent way of seeing parts of South Devon.