We have a trip to Cornwall and Devon for July. We are staying 2 nights in Exeter then renting a car and heading down to Truro.
Here is some information that I copied from others on this website.
Don't know about Plymouth. I stayed in Exeter and Devon for one night each (last-minute booking challenges in the coastal towns) and enjoyed them both. I find I always like the non-coastal places because they're less touristy. My two coastal stops were Mevagissey and St. Ives. The latter is one of the busiest, most touristy coastal towns but food for visitors interested in art and ceramics. Mevagissey is small but still has traffic issues. I suspect traffic issues are a given in Cornwall in July.
I believe the guild hall in Exeter is a major monument, but I didn't get to see the inside.
I liked both the Eden Project and the Lost Gardens of Heligan. They're close to each other, but both are quite large. I think trying to do both on the same day would just result in paying two (rather expensive) entry fees and rushing. But I when I pay to go inside a tourist attraction, I do tend to want to walk on every path, etc.
My general tip for the entire area: If you want to eat your dinner sitting down in a food establishment (as opposed to consuming fish and chips on the jetty), you had better make a reservation somewhere. I'm not sure about lunch, because for me that was just something I grabbed.
As far as things I enjoyed, I, too, loved Lost Gardens of Heligan. Didn't do the Eden Project but we did a china clay mine tour at Wheal Martyn just outside St Austell. I'm not big on mine tours but it was more interesting than the Levant or Geevor Tin Mine we also did. (Geevor is also on the Rick Steves Villages of Southern England tour).
I liked St Michaels Mount but it is pretty steep and slippery up to the front door of the house. The tides here are interesting and it was cool to watch the walkway out to the isle be covered with water and then watch it recede.
I thought the Iron Age Chysauster Ancient Village was quite interesting. It also has a neolithic Fogou which is an underground passage.
I didn't much care for St Ives but it was school half term and it was jammed with families so more people around than I like. I did like Mevagissey as well as St Mawes (you can take a ferry across from Falmouth), Charlestown, Fowey and Boscastle. I've visited Port Isaac twice and while I was excited the first time because I'm a Doc Martin fan, really, it's terribly crowded and you can see everything in about an hour.
We stayed in Plymouth one night on our way to Cornwall. We enjoyed the Barbican area and Plymouth Hoe area, the Mayflower Steps, and Sutton Harbor. The view from the top floor breakfast room at the Holiday Inn on Armada Way in Plymouth is wonderful with the ferris wheel and waterfront. We also stopped in Truro for part of a day. The cathedral in Truro is beautiful. Another place we absolutely loved was the Lanhydrock National Trust Estate near Bodmin. It was well worth a stop. Have a great time in Cornwall!
Mousehole is a wonderful village to visit, if not stay in.
Loved Minack Theatre, tintagel, st michaels mount and just driving around Cornwall, which many feel the opposite about.
Dartmoor - Roam this distinctive craggy landscape and take a glimpse at the Dartmoor ponies grazing in the shadow of the granite outcrops.
Exeter - Stroll through this ancient Roman city, from the magnificence of its gothic cathedral to the city’s medieval underground passages.
Boscastle - One of the last remaining unspoilt harbour villages in the British Isles, the medieval harbour and its charming fishing boats are a joy to see.
Tintagel Castle - This ruined castle full of Arthurian legend is perched on cliffs above an immense cave full of magic.
Land's End - This iconic location is lined with coastal paths and cliff trails.
St. Ives -