Daily tasks
You could have daily tasks with rewards such as res/cash/pp like:
Capture 3x level 9 oasis,
Kill 2000 of any troop,
Sell 30k's worth of goods on the market,
Hunt over 20k cash,
Train 1000 cats etc,
These could be extended for weekly and monthly tasks that offer greater rewards such as coins or free VIP or enhance on city mines.