From our friend's description, it sounds like the event can be as formal or as laid back as you like. Her's was fairly informal, with friends and family dropping by between set hours in the afternoon and a buffet of foods for people to snack on. The baby was definitely cooed over, but there was also just a lot of catching up with old friends, gossiping, and general merriment.
We did some poking around to see if there were specific foods or drinks traditionally served at Sip 'n' See's, but our findings were inconclusive. The main requisite is that there be something to, in fact, sip. Historically, this would have been coffee, tea, or punch (spiked or otherwise!). The edible spread seems to include the whole range of quick breads, cold salads, casseroles, and sandwich fixings.
What's your experience with Sip 'n' See's?
Related: Is Sweet Tea Going Commercial?