Main activities are doing overthrows in the pale heart and the seasonal activities on Nessus/Helm. Alongside the usual activities for getting powerful/pinnacles to up your light level. If you have a fireteam there are new missions that require co-operation (cooperative focus missions and an exotic mission).
Also look for the pathfinder node on the pale heart and in the ritual activities, this largely replaces the bounty system and is another source of prime engrams and the Ergo Sum sword.