I live in Amsterdam. I suggest you buy Van Gogh museum tickets online in advance, there are always long lines there and I believe you can go use the faster line with a pre-purchased ticket. Also better to go early in the morning otherwise it might be too crowded.
For me the must see in Amsterdam is Vondelpark, great for a picnic on a warm day and in the summer the ambiance is somewhat hippie/Woodstock. Another suggestion is the markets (called markt here), Albert Cuypmarkt is the one all the tourists go, it's ok but for a more local experience try Ten Katemarkt, which is also next to De Hallen (which is new and has a food stall/bar market along the lines of San Miguel in Madrid and similar markets). Finally I like the Eye Museum (film museum and movie house) which you reach by free ferry from Centraal Station, great views. And the very modern public library, 7 floors with a nice self-service restaurant on top floor and also fantastic views of Amsterdam, free of course. The library is also very close to Centraal Station.
I've also visited Stonehenge but it was a uni trip and the coach took us directly to the site. There appear to be direct trips from London which would be less of a hassle than the train+bus combination, such as:
http://www.premiumtours.co.uk/tours/...ours.id10.html
There is a paved path at Stonehenge and a fence around the stones. Here is the Stonehenge website on disabled visitors, seems you would be able to get free entrance as the carer:
http://www.english-heritage.org.uk/d...access?lang=en