Giggle 2 myself or Giggle to myself. lol is overused and tends to be too strong of a phrase in most cases. G2M is appropriate for those times when you are IMing or texting...and something makes you laugh, but does not necessarily ensue hilarity.
IMing your co-worker
"Did hear that Justin is late again?"
"Seriously? What's his excuse this time?"
"Yesterday he said he broke his left pinky. Today, he said his he forgot how to get to work."
"That's so great - g2m."