The distances aren't very great, but in some of the more remote coastal areas of Cornwall, the roads are really narrow and winding (often single lane with passing places).
The traffic is heavy, especially in the summer tourist season, on all the roads, but progress is still reasonable on main roads like the A30. Look at the numbers of the roads when you zoom in on Google maps. A-roads are the more major ones and the traffic usually flows okay away from the town centers. B-roads are smaller but still usually two-lane. It's the little windy roads without any numbers that get really jammed.
slim's suggestion of putting the places you might want to travel between into Google maps is a good one. Remember to put the likely start or arrive times in too so that you get a good estimate of the likely delays caused by traffic.
How are you intending to travel? Hire car, public transport, or what?
There are a few tourist traps such as Land's End. The coast is beautiful there but no more so than lots of other places - however it's overrun with too many tourists and there are ugly tacky gift shops - my advice is to go somewhere less famous and spend some time exploring on foot, or maybe hire a bike.
I recommend:
- Lynton/Lynmouth (all the same place really) on the north Devon coast - it's a steep coast with cliff railways and such and the villages perched on what land is buildable
- Exmoor and/or Dartmoor National Parks if you like open wild places.
- Plymouth if you're interested in naval history and ships
- Torquay or Newquay if you want to see traditional (though crowded) British seaside resorts with beaches and such
- The Lizard heritage coast (area around Mullion, Lizard, Cadgwith) if you want to see coastal features similar to Land's End, but with less overcrowding