Oxford or Windsor for 3 nights

Avebury needs a car, as does Stonehenge. There are other sights on Salisbury Plain which need a car.
Oxford and Windsor are best done on foot. What you need is an hotel in Oxford or Windsor with parking space. Ensure the hotel has parking before booking and and you will be OK.
It is worth visiting the city of Salisbury, but again you need to find parking and continue on foot.
If you organise it right and have your visit to Oxford or Windsor at the beginning or end you could save 1-2 days on the car and save some £££ (not $$$ - wrong currency).

Thanks Chris. I've already been to Stonehenge and Salisbury so don't need a car for that reason. This will be at the end of my trip to Wales and I will have had the rental car for 2 weeks, adding the 2 extra days seems to cost so much more per day than the weekly rental. Which is why I was hoping to get rid of the car. Would it be that hard to get to Avebury from either Windsor or Oxford using train/bus/taxi - whatever? Or would that end up costing more than a day's car rental? I know I can train between Windsor and Oxford easily and cheaply. Maybe I should rethink my itinerary. I'll be coming from Cardiff by car and planned to stop in Gloucester to see the cathedral. Would it be feasible to add a stop in Avebury to see the stones on the way to either Oxford or Windsor?

Avebury is not really that convenient to Windsor or Oxford. I have been to Avebury and while it was interesting it is not easily viewed as a monument from the ground. Windsor is really a suburb of London right near Heathrow. The closer to the city the more spendy the rooms. Once you've seen the Castle or Eton? Oxford is close to Cotswolds and is interesting for seeing the famous museums, etc. I would stay near Woodstock which is by Blenhiem Palace and go into Oxford on the bus for the day.

I can highly recommend B and B Frances' Lodge in Windsor. They have ten rooms, all very modern and comfortable. The single is on the ground floor and costs £70. Breakfast is outstanding, with offerings like Eggs Benedict made with free range eggs and smoked salmon instead of ham. The location is excellent---just a short walk from the castle and the river. We would happily stay there again.

Via Michelin says it's 49 miles, or 1 hour and 20 minutes, from Gloucester directly to Oxford, and it's 100 miles, or 2 hours and 22 minutes, from Gloucester to Avebury to Oxford. The times are probably too short, but it looks like it would cost you about an hour to make the trip via Avebury, plus of course your time at the site. Maybe that's your best use of the last day with the car. Then stay in Oxford (much more to see than Windsor) and make the Windsor visit by train. Don't know your next move, but if it's Heathrow then Windsor could be your last stop before you fly home.

Thanks for the comments so far.

@Dick - this is my last stop before going to Heathrow to fly home. I know Windsor is closer but there is bus service to Heathrow from both of them so that is not a big issue in making the choice.

I'm happy to hear any more opinions - the more the merrier.

Avebury is possible to get to by public transport - there is a bus each hour from Swindon railway station, but this is backtracking some distance.

I think I'd prefer to keep the car for more flexibility. The savings or extra cost depend on how long you're already keeping the car. Weekly rental rates are often a good deal, with 5, 6, or 7 days all at the same price.