keeping teenage grandchildren busy

Give them your iPad, your GN password, let them loose
Oh somebody has already done that!! (AllYoungPeople)

Seriously English Heritage often have special interest days with a historical theme, or you could do a treasure hunt (motorised if you like)
Look for things like birds of prey open days, how about old fashioned picnics (sand in the sandwiches, tea in a thermos) Space Centre, seaside, "sights" of London, natural history museum, or fossicking for fossils on the Dorset coast, if that is your part of the world. Check out your library if you still have one for adventure trails, or special events in your area.

They'll have their tablets and phones with them. That's all the entertainment teenagers need.

If you have a local sports centre, how about hiring a badminton court or short tennis if they get a bit "stir crazy!" Indoor climbing centres are very popular here and I have seen kids there from about five to late teens! Are there any cycle hire facilities and trails near you? If you have a local country park nearby there are usually plenty of activities there. Swimming pools sometimes have "splash" days with inflatable obstacle courses that are great fun. Give them a ring and see what their summer holiday programme is. I wouldn't worry about it too much - they will have their own ideas about what they want to do, and their parents will control any propensity for spending too much time glued to tech! I hope you have a lovely visit with them - precious times!

I think one has teenager has been so bored and unable to sleep last night that s/he (probably the latter) has found his way onto GN!

Not recommending it as a way to occupy bored teenagers but at least it shows some initiative - doesn't it

Thanks for the ideas so far. Yes they have their tablets and phones, but their parents try to curtail their use.
We're in France so no possibility of all the London activities. They do enjoy that kind of thing. The last time I was with them in England we had a day at Bletchley Park which we all enjoyed.
I've just remembered there are 2 outdoor swimming pools nearby.

Give them your wi-fi username and password for their tablets and they will find plenty of ways to amuse themselves. If they're able to use Skype (or equivalent)they'll be able to speak to their friends and compare their boring holidays!

That's it - annodomini I can do that. They will miss their friends.
When I see them in England in the summer - they work in Kuwait and have a flat in England for the summer - they spend most of their time with teenagers in the other flats in the converted old house, or from the school in Kuwait that they attend.