I keep a supply of envelopes (for folding an A4 into 2 and into 3) and stamps for domestic and overseas postage ready for answering bills or queries, or sending cards etc. I also keep a selection of birthday, get well, thank you, accept/regrets, thinking of you etc cards for last minute invites or rememberings, or just deal with things immediately they happen.
Less day to day but helps keep me sane when we can often have visitors announcing their arrival (or DH reminding me of an arrival he agreed but never told me) mid way through working day. I strip the guest bed once any visitor leaves - and then immediately make it up fresh. So if visitors DO land unannounced, I only need to put out fresh towels. (I need to get back to that as we are au-pair-less for summer so likely to see lots of visitors again).
I also keep a range of travel sized toiletries for same purpose. With a couple of brand new toothbrushes. Male and female suitable. (I always had a few, but I made sure I had everything that might be necesary when my grandad was very ill 2 years ago, as siblings would be coming from overseas at short notice to visit, and when he died, so it was better to have things in case they forgot something and remove stress for them).
When my Grandad was ill that time, I also made the "Automatic Pilot List". To turn around as soon as I heard news and organise things to deal with everything. From a full shopping list of things needed specifically for that event (feeding hordes in my grandparents house and also my own, as well as just keeping DH, DD and I going too), to what order to do the cooking in (I had worked out double batches of 2 different main meals, soup, cookies, apple pie and scones for baking, and lighter meals and snacks), household jobs that would need doing (like cleaning bathrooms), making sure beds were made and what options I had for extra bodies, what supplies people might need (from first aid, brylcream, safety pins or tights, to notepaper or cards and pens for prayers) and making sure I had stuff for overnight visitors but also anyone stopping at mine from airport or whereever to change, and making sure I made time to talk to DD as well. The reason for that was to make sure that when the time came, I could just go onto auto-pilot mode and follow the list regardless of my state of mind. (Especially as things tend to happen very fast here). I have updated it recently as my 2 granmothers are both getting extremely frail. I didn't need to follow it slavishly, but it did help working through everything beforehand, and having something to refer to when I knew there were still things to be done or people offering to do something.