My recent fave find is Napa cabbage.
I've had it in the past but recently started buying it to add to my salads. I buy the bagged lettuce blends and love them but sometimes they go bad/slimey on me really quickly. So instead of buying two bags of the mix, I buy one and also get a head of Napa and chop it and keep it in a Ziploc bag in the fridge. I mix the Napa with the bagged lettuce when I pack it for my lunch. It stays fresh all week and helps me extend the bagged lettuce and keep from having to buy two bags of that and possibly having to throw some of the lettuce out.
P.S. I had worried about the cabbage causing gas issues (), but so far haven't noticed any problems.