exploring Cornwall

Perhaps the first thing to say is that Dartmoor isn't in Cornwall, it is in Devon. Although the counties border each other, they are quite different in character and Devon is easier and quicker to get to from London. Dartmoor is a wonderful place to visit and there is some good walking. There are plenty of places to stay on and around the Moor, and Exeter isn't too far away.
If you go into Cornwall, it is best to stay on the coast. There are excellent walks on the South West Coast Path, which goes all the way from Dorset round the peninsula to Somerset.

Although not in Cornwall, Dartmoor National Park is beautiful. St. Ives is a nice place to spend a couple of days, but we weren't all that impressed with Penzance.

Years ago I stayed in a small B&B where the landlady told me how she loved walking on Dartmoor. But, she said, watch out for the ponies because they bite!

I think 6-7 days is not enough to include the Scilly Isles, unless you just focused on (1) London and (2) Scilly. The train to/from Cornwall takes the better part of a day.

We have been to Cornwall several times. Our last trip, June 2015, heading from LHR, we spent one night in Salisbury, five nights in Cornwall, and one night in Wells before heading north towards Scotland. You really need a car for Cornwall if you want to poke around in the little fishing villages and visit gardens and the many megalithic sites. Since Cornwall is a long skinny peninsula, it works best to stay in two locations to minimize driving. Don't think north coast and south coast; instead choose a location at the east end of the country and one at the west end of the county. We chose Mevagissey, which worked well for Fowey and pretty fishing villages to the east and also for St. Isaac, Tintagel, and Boscastle which are on the north coast which is only about 25 miles away. For the west coast, we chose Gunards Head which is adjacent to Zennor. If you have a car, I would avoid staying in either St. Ives or Penzance - both are just big enough to be a pain for parking and driving - St. Ives can be visited as part of a day trip. Mousehole is charming - one of our favourite spots. Wherever you pick to stay, be sure that it includes on site parking.

We stayed at a fisherman's cottage in St. Ives and drove around the south (5 nights) then we drove north and stayed at a B&B in Bude (3 nights) to do northern Cornwall, Tintagel Castel, St. Issacs, . There are so many things to do that staying in St. Ives was a good decision as we had purchased a week car lot rental as no real parking in the village, at night we walked along the main drag and ate, did a pub, talked to locals. Went to Lands End, St. Michaels (tide was in so no walk over had to go on boat), Mousehole, old mines, cliff walked, seal watching boat ride, ate at a Rick Stein restaurant in Padstow on the way to Bude, son surfed/played golf in Bude (nice beach). Train is not advisable as really need a car in Cornwall. It takes the better part of a day to drive there, we paid extra for an automatic as husband didn't want to do the stick shift and with the small roads and hedgerows it was a good decision. With a car you can stop in little villages along the coast,

I agree with the person above who said that 6-7 days, including travel, to Cornwall isn't enough to include the Isle of Scilly. I looked into it when I was there for a week and I couldn't fit it in. The boat trip is slow too, so if you only went for a day, you would only have a few hours on the island before having to catch the boat back. Flying would be the better option for a day trip.

My favorite parts of Cornwall were the town of Mousehole, Tintagel (which you will love as a hiker), and just renting a car and exploring. Be sure and check out some of the other recent threads in the forum about Cornwall as well, good info in there.

Mevagissey is well worth a visit, it's beautiful!!!!!

Agree with TC comments above. St. Ives is a good home base. Mousehole is adorable, but maybe a little limited for a home base. Penzance seems more like actually where people live/work, not interesting for tourists. Dartmoor is well worth it, but as others have noted it is a good drive from Cornwall. Check out Rick's upcoming shows (releasing this fall I believe) that will cover Dartmoor and Cornwall. You will need a car to do this however. Public transport and trains won't get you to the most interesting parts of Dartmoor or Cornwall, at least not easily.

Another vote for Mevagissey. The Buckingham House B&B. My sister and I used it as a base several years ago and it was perfect. Loved it so much I went back again two years ago..Oh my goodness could it possibly be?? ..with my best friend. Watching people trying to drive thru the narrow streets was one of my favourite things to do. Things to do: I really liked the Lost Gardens of Heligen and of course The Eden project is not far either. We went to Jamaica Inn but wouldn't do it again. We also spent one night in Princetown in Devon and visited the prison where you can sit in the electric chair.

I'm also looking into options for visiting Cornwall next year and learned there is a high speed train from London but it will still take the better part of a day getting there. For some inspiration on seeing northern Cornwall you may want to check out the TV series, Doc Martin. It's filmed in Port Isaac and the scenery in that area is stunning.

On two visits to Cornwall we have stayed in Penzance (without a car) and Boscastle with a car.
I agree with previous poster, if you are spending a week in Cornwall, a car would enable you to fully enjoy and appreciate this lovely county.