Manuals for any appliances staying there (if you still have them) - boiler, in-built cooker, aga, solid fuel stove, air conditioner, in-built stereo system, alarm, solar panels...who knows what anyone has in theirs. And info on any lights that need special bulbs, or anything that is not quite "standard".
If you have any details for the last servicing of same, useful too. Or installation details that might be useful.
Agree anything about specialized locks (we have a card to get front door keys cut too).
If you know them, the key for each room hanging in it's lock. Any spare keys that you know belong to the house put together. And any window locks too.
Any spare paint, tiles, wallpaper ends etc for touching up.
If you have a few favourites, the leaflets of local takeaways, good local taxi company, local handymen (plumber, electrician, gardener, window cleaner, chimney sweep etc), perhaps a note of any shops (especially like a nice butcher, fishmonger, greengrocer or bakery) that you have loved and found to be good service/value/produce etc, especially if they are not terribly obvious on the high street.
Details of bin collections, and what goes in each bin if you have multiples! (Our green bin is green, but our brown bin for compost is the small green one, and our black bin for landfill is purple - neighbours have green, brown and black as use a different company).
Details of the energy/water providers/phone/broadband/tv services you had signed up to - they don't necessarily need to stay with those in some cases, but it helps to know these things to arrange handovers and potential changes.
We had some spare personalized notepaper - it just had the address but not our names printed on it (our great idea to not get "Thank you" cards after the wedding - write everyone a thank you letter instead and then have leftover paper to use ourselves), so we left that and a decent handful of the envelopes (I kept the leftover plain paper and the rest of the envelopes).
If you know they have children, useful numbers for sports clubs can be handy. Or for the school, phone and email contacts, principal's name, secretary's name, PTA contacts, etc - if there is only 1 local option and you know them yourself.
Emergency supplies - YYY to loo roll, bin bag, bottle of wine/box chocs, congratulatory message (card or note) etc.