How to get from Heathrow to Bath and back to London?

I think that's a crazy itinerary trying to pack so much into a few short days ... you'll spend all your time travelling and have little time left to actually see anything

anyway, yes hiring a car would be best but if you're American then driving on the "wrong" side of our congested roads would be challenging for you ... if you decide to do this make sure you read up our Highway Code before you start

http://www.direct.gov.uk/en/TravelAndTransport/Hig...

with a car you're more able to get to out of the way places and travel when you want to (which would make stopping at Stonehenge a whole lot easier) ... also you can stay at more reasonably-priced out of town motel places like Travelodge, Holiday Inn etc ... if you're hiring a car it's well worth getting one with satnav ... however drop the car off before coming into London, a car is a liability here (congestion, very expensive parking)

Heathrow to Bath by public transport ... take the Railair coach to Reading and the train to Bath or Bristol

http://www.railair.com/

http://www.thetrainline.com/default.asp?href=&T2ID...

and the trains back to London are fast and easy

getting to Stonehenge by public transport is tricky ... see

http://www.stonehenge.co.uk/arrive.php

Go by train; get a europass.