Ok, I've found a decent list, here are the secret achievements. Note that all of the structures must be on the map at once to gain the award (you can't build 14, demolish one, then build another).
Hot Dogs Fan: Build 15 hot dog stands Simple Solution fan: Build 15 cabins Trifles Fan: Build 15 gift shops Spa Salons Fan: Build 10 sunny centers Mega Party Goer: Build 10 disco Aristocrat: Build 3 golf clubs Casinos Fan: Build 5 crystal casinos Altitude Lover: Build 2 galaxy restaurants We Remember about the Defender: Build 2 gold defenders Amusement Park: Build 2 observation wheels Great Pyramids: Build 3 pyramids Lord of Excitement: Build 3 pearl casinos Master of Bowling: Build 3 Bowling clubs Following the Montgolfier Brothers: Build 3 Ballon launch platforms The Sun Only: Have all power generated by solar stations, remove all windmills
Go to the social button on the bottom right of the screen. Hit the friends tab, you will be taken to their island. Above three of their facilities you will see a cleaning brush. Tap each of those to receive money.
Best Answer
I checked out wikia.com and got this from under the game Paradise Island.
Great Administrator
Improve Administrative Building at least to level 50.
The rewards are 3000 XP and 15€.
You can visit the link here : http://paradiseislandhd.wikia.com/wiki/Awards