In Taiwan we also use 顆顆(or科科) or use Roman Letters "kerker", or even bopomofo "ㄎㄎ"
This is like 呵呵, a milder laughter, or we could say a chuckle.
The "k" sound here is like the mild glottal sound accidentally made when chuckling.
Note that 顆顆 and 呵呵, in Taiwan, may seem insincere to many people, like you don't want to respond with actual words so you instead do a chuckle.
Sometimes 呵呵 can be even considered impolite, like you are saying "Haha, really funny" in a sarcastic way.
嘻嘻(heehee) is another word for laughter. It sounds a little naughty, cheeky, etc., so it is mostly related to practical joke or something like that. It could be a giggle or even a snicker. (嘿嘿 would be stronger than嘻嘻, which is more of a snicker than 嘻嘻)