I always try and schedule hot tub parties closer to my water change outs. Your water will be a mess the next day.
No Glass is a good rule. Invest in some decent plastic tumblers.
Have plenty of Towels ready. I also like to provide my guests with robes. If you can having a place for people to change and put thier cloths is nice idea, rather than piles of cloths on the bathroom floor.
Folks might need some plastic bags to take thier wet suits home.
A cooler within reach of the tub is nice for keeping refreshments close by.
Tiki tourches add a nice touch, but you've got to be VERY careful. but i love em!
New Years Eve I lit off a few very large firworks display. Sitting in the tub at night watching the fireworks was very nice. However, keep in mind the "Fallout" from fireworks can land in your tub. Far away with a leanght of cannon fuse works well. (If you got neighbors, either be respectful of them, or better yet; Invite them!)
Careful getting in and out of the tub.
If your guests are not used to drinking and tubbing, watch out. Folks can get in trouble very quickly.
With a bunch of folks in the tub chatting away, music becomes noise. If you are looking for a quiet soak with some close friends then backrougnd music is nice.
An outdoor fire nearby with seats is always nice. (I do this in the summer. Too cold to be in wet suits around the fire in the fall).
Let your guests catch thier breath and relax for a bit before sending them home at the end of the party. After a nice soak folks like to relax) usually have coffee for them in the living room, by the fireplace, in thier robes.
Don't forget the cigars.
More thoughts:
Winter hot tub parties? We've had races around the house in the snow storms as well as folks making snow anglesand then jumping in the tub.
This is "invigorating". Yea right. still a lot of fun though.
Oh yea, keep an eye on the "empties" People when finished with thier drinks tend to reach down and place bottles and cans by the tub's base. These always get knocked over. a small trash bucket for empteis is a good idea. A tray for empy tumblers is handy.
Folks will tend to sit on the filter cover. You might want to keep an eye out for this. It can break and hurt someone.
Food while in the hot tub is a bad mix.
The Hot tub is usallly at the end of the evning, after dinner or what have you. It tends to make folks sleepy.
Keep the water temp on the low side. Most folks liek to show off how hot their tub can get, but this just makes guests tired or uncomnforable and get out quicker.