Get out of London or do daytrips?

Devon is a long drive, especially with kids. You've got shorter daylight hours and possible iffy weather. Why do all that travelling?

define 'exciting'...?

if an hour at Stonehenge is really that important, do a daytrip. You'll find it much quicker nad easier to get to Paris by train from London - otherwise you are looking at one way car hire which will cost a fortune.

Google maps says under 3 hours from Salisbury to Devon - is that accurate? If so, thats hardly anything. Perhaps travel times are relative to where you are from. Where I am, we often do 7 hour drives to visit the borther in law, or 5 hours for Ikea, etc.. etc...

Where do I begin?

The consensus on this board is that Stonehenge is best avoided: Avebury offers a far more rewarding experience.

Devon (basically where I live) is not only a long way from London, but also rather large. So you need to plan in a little more detail if anyone is to give a helpful response. And don't forget that if you visit Stonehenge or Avebury you will be close to Somerset, which is arguably at least as attractive as Devon: I would rate all of Bristol, Bath and Wells as five-star destinations with different plus points.

If you wanted to get from that part of the country to Paris, the most obvious route would be a flight from Bristol. Alternatively, you could get to Poole or Bournemouth and have a very pleasant ferry-ride, taking a long time but stopping off in the Channel Islands. The journey from the port to Paris would be significantly shorter than that from Roscoff (which would be a pain since you need to take a bus to the nearest rail station and then have a very long journey). And the ferry to Roscoff leaves from Plymouth, which is the far side of Devon (almost Cornwall) so a long way from Stonehenge...

• Stonehenge sucks, Avebury is much better
• When hiring a car, you have to return it to the place you got it from
• There should be more than enough to see and do in London. "Why, Sir, you find no man, at all intellectual, who is willing to leave London. No, Sir, when a man is tired of London, he is tired of life; for there is in London all that life can afford."
  — Samuel Johnson

• There should be more than enough to see and do in London. "Why, Sir, you find no man, at all intellectual, who is willing to leave London. No, Sir, when a man is tired of London, he is tired of life; for there is in London all that life can afford."
  — Samuel Johnson

True: one could spend a lifetime in London and not run out of interesting things to see and do. However, most people would prefer to visit Paris as well, and by the same principle it makes sense to experience those parts of England that are very different from London, but equally fascinating. My considered opinion is that Devon is more different from London than is Paris, and experiencing these different faces of England is fascinating for the visitor.

be aware of UK traffic. Could take 3 hours, could take much more.

up to you, but travelling to another country and then spending loads of precious holiday time in a car never makes much sense to me.

If you wish to do a 3D2N trip, I would look at all or some of these:

London > Salisbury Cathedral > Avebury > Southern Cotswalds > Blenheim Palace > Oxford > Windsor

And back to London. If the boys are curious about the world, then they will get a lot from it too.

Don't underestimate the traffic ... note there is also a Congestion Tax within London for private vehicles including rentals (it's nice to return just after 6:00 pm). I would also catch the EuroStar train to Paris from London - if you pre-book it there are cheaper tickets available.

We stayed in a very nice hotel/guest house called the King & Queen at Longcot (between the Cotswolds and Oxford) ... recommended.

While the ferry would be more interesting, as far as getting to Paris goes I reckon it's just going to be more sensible and time-efficient to either return to London and fly/Eurostar from there, or take a flight from one of the airports in the southwest - Bristol or Exeter.

A flight from an airport in the region would certainly be more time-efficient... Sensible? Not if they want the experience of the boat-trip and to see some of the many attractions on the way (St Malo; Mont St Michel; Normandy beaches; Cherbourg; Bayeux Tapestry...)