Assuming you’re cycling…
You could head up to Dartmoor – cycle through the city to Plymbridge Woods then head up to Clearbrook, then vaguely in the direction of Burrator reservoir – you should be able to find some quiet sheltered spots around there.
Or, if you fancy some sea air, you might be able to camp on the beach near-ish Wembury (to the west of the main beach) or Mothecombe, but you will have to be a bit a smart with timing to avoid people (especially at Mothecombe, where there is an old chap who sometimes patrols the beach – technically a private beach with public access a few days a week, but there’s a public road down to it and the coast path runs past). Both beaches have a bit of sand even at high tide, unlike if you head over the Torpoint ferry and go around Rame Head / Whitsand Bay. The A379 into the South Hams is a nasty busy road though, so watch out if you do go that way. You could try and camp along the coast path in either direction but you might struggle to find anywhere secluded and sheltered enough.
You could also get the Cremyll foot ferry from Royal William Yard to Mount Edgcumbe (65p extra for a bike!) and try and find a spot in the country park – it’s lovely but I doubt they look fondly on campers so you’d need to be extra sneaky.
