No experience yet as still recruiting but I'd say write it into your intital letters etc. I've put this:
We would expect that you might eat with us 1 or 2 times a week and join in occasional family trips. However, as a host family we would normally expect to have some private time to ourselves, particularly in the evenings. During this time, our au pair might retire to his or her room to watch TV or study, or go out with friends from language school.
I do realise that an au pair might be young and lonely - although we are activly trying to recruit someone who is outgoing and who has been abroad before.
On the other hand I don't expect any teenager or young adult to want to spend their entire time with an older married couple!
And I don't think tis out of line for us to want some alone time as a couple.
I would include an au pair on weekend activites involving DS but if me and DH are going out I don't think we'd want a 19 yr old playing gooseberry! And sometimes we will want time just as a family as well, me DH and DS.
We're offering a TV/DVD + freeview, broadband wireless internet in the au pairs room. I'll also show them around clubs, theatre, pubs, cafe's, bowling, live music venues, cinema, swimming baths, gym, ice hockey arena (we're looking for a canadian) etc that are all open in the evenings and all accessible within a 30 minute walk of our house.