Traditionally just plain hot water, so that the guests can be sure that the water for tea is good quality. These days sometimes flavoured with 塩桜・梅干し・toasted rice.
Winter
甘酒 is traditional for very cold winters, sometimes even with an egg and ginger added.
葛湯 could be a nice idea (just 葛 and 和三盆, maybe also some grated ginger).
If you want to be more Western, a hot toddy might be nice (being the water of life after all).
Spring
梅干し for February, of course
塩桜 for April, of course
Summer
KMR25 used kefir in summer, which is a great idea.
For 1st of August, I used Edelweiss cold brew. It worked very well, and I think alpine herbal tea blend cold brews in summer are a good idea.