I'd have to agree with Stut, Stonehenge is definitely over hyped and not worth going out of your way for.
If you do go to Stonehenge, Bath is nearby and well worth a visit, the Dorset coastline and places like Poole and Lyme Regis are within reach and worth a visit.
If you go to Stratford, Oxford is worth a look as it's on the way from London.
Dont take your hire car into London. Collect it once you have done London, parking costs a fortune and public transport is quicker and easier.
There are a number of places you can visit from London in about an hour by train, Brighton, my home town with its Royal Pavillion being just one example.