Polygon picked up an interesting thread on the
Destiny sub-Reddit in which gave tips as to how to get Strange coins such as:
Weekly Heroic (9 coins on lvl 30)
Blue Engrams (ROC Strikes)
1 Strange coin for doing the Draksis mission for the first time per character
1 Strange coin for getting the gold Chests on Mars per character
The daily gold public event package at the Postmaster
Faction level-up packages
Whilst I would avoid the Faction one unless you're an avid Crucible player since they give you crucible marks rather than Vanguard and the ROC playlist is only available to those with the DLC, the public events and the weekly Heroic are definitely worth pursuing.
If it's ascendant materials you're after then the Daily Heroic story mission and again, the public events are the ones worth doing.
If anyone is level 27 (the game allows players three levels below the mission level to join) and wants/needs strange coins from the level 30 weekly, then give me a shout as I'll be happy enough to help out.