Are you planning on having it spread throughout the day or all guests at once? IMO as a guest it's nicer when a Sip & See or Meet the Baby is spread out. That way I don't feel like the new grandma/aunt/whoever is waiting for a turn while I hold the baby for what feels like only 30 seconds.
If you're spreading it out, I would also suggest keeping a really simple theme (or no theme). If you feel you must decorate (again IMO, not necessary) pick a color and the first intial of LO's name. Have light refreshments such as finger foods, punch, and mimosas. Cookie trays are awesome for sip & sees too!
If you'd rather have all guests at once my biggest piece of advice is to have a start AND end time. Have someone (new daddy) enforce the end time for lingering guests.