Altars of sorrow are pretty nice since there're plenty of ads and most fall over to any damage. Onslaught is also fun and you can earn some pretty great weapons (don't forget to attune at zavala if you want to target something). If you don't want to compete with other players then the opening of grasp of avarice is also pretty quick.
Overall would recommend weaving in solstice bounty completion with other things you're trying to get done (catalysts, event challenges, etc.) since farming almost anything like this can get monotonous