1. 'nother list item. Might be too late now for after season sales on this, maybe next fall. You want to start collecting your set of outdoor dinnerware: plates, cups, glasses, coffee mugs, beer mugs, bowls (multiple sizes, for place settings and serving), napkin holder (anti-wind), salt and pepper shakers, wine glasses, etc, all plastic or wood (no glass at the pool). Cleaning up broken glass is pretty tough on outdoor decks. You can sweep and blow and vac all you want, but it'll be the youngest, tenderfooted-est little one that finds the tiny shard you miss. Best to stick to the no glass (NO exceptions) rule, but you have to be able to offer all the alternatives. No wine bottles or beer bottles either (those are the toughest to enforce). I dedicated an entire kitchen cupboard to my "pool collection" and steer guests to it.
2. I also get great use out of a large, rectangular bamboo serving tray, which I use to bring out large collections of dinnerware, or food for serving. Saves a lot of trips back and forth. Bamboo is a good material for outdoor wear: really tough, washable, and not so, well, plastic-y.
3. Did someone already suggest? A garbage pail near your sitting area. I just drag over my garbage can, but a nice-looking smaller pail or bin would be nicer. Suncast probably makes those, too.
4. Do you have bugs? Cintronella candles, or torches, or oil lamps. Or a bug zapper. (I have 'em all.) Bug cream for your sensitive guests.
5. I keep a roll of paper towels and cleaning liquids nearby my outdoor area (in the BBQ cabinet), along with an old set of hot mitts.
6. You mentioned string lights. Nice. I took the trouble to install a nice looking hanging light over my outdoor dining table. I like to see what I'm eating (especially outdoors!), and we use it for when we set up the board games out there. Dimmable, of course, to set the right amount of light for the activity. Others mount fans as well, which might help with bugs, too.
7. I wired a second door bell to my existing one and mounted it near the pool. I can't hear my inside one when outside, and your late-arriving guests will appreciate not getting ignored at the front door (even if they weren't polite enough to show up on time!!). I think they have wireless solutions for this, if you can't run a wire.
8. Ugh, I had another but it leaked out of the ol' noggin before I could write it down. Oh well, more later...