Most of traditional music are pretty simple and they are great in camp fire jams. It's nice to know how to play some Deep purple or CCR to jam with friends but if people (or family) know you play guitar they will ask you to play something at one time or another.
Try to play smoke on the water to your grandma ;)
My suggestion is to learn 4-5 folk song that you can play anytime. That way, if people ask you to play, you'll have a couple of easy song that everybody knows and can sing along with you.