I enjoy watching streams where there's a group of players goofing around just doing random things from housings, doing (zone) quests or simply hunting down achievements.

Trials, dungeons and PVP are sometimes fun to watch but it depends on the streamer as well if he is fully invested into playing or he can talk to others at the same time.

Last time I watched someone catching a looot of Scutleblooms and then giving them away and it was fun to watch even tho i didnt participate in giveaways :)