A sling or baby carrier is it. Maya Wrap is the one I mostly swear by, but once baby is over about 15 lbs. a mei tai/asian style baby carrier is better until they're really walking a lot and wanting up/down constantly (when the Maya again becomes ideal). If I truly had to make do with only one, it'd be my Kozy Carrier (ABC).
A car seat since I live in Lousy Public Transport, USA.
A blanket on the floor (beofore mobility) or random baby gadget (bouncy seat, swing, etc.) to set baby in during those times it's really necessary to put her down (like when I'm in the shower).
