"Hee", "Hee, hee", "Hah", "Haha", etc. all work fine. "/me chuckles", "/me snickers" and "/me giggles" also work fine.
For heavy laughter, there's a "XD" smiley, or "Can't type... laughing..." or "Hee, my sides hurt..." or (my personal favorite) "/me dies laughing" or "/me falls on the floor/out of my chair", etc, etc.
I also knew one fellow who used to type random letters for a while when laughing hard... the intended implication being that his fingers were getting tangled while trying to type while laughing.
Peace & Luv, Liz