Airdrop and bounty program use to promote some ICO or existing token/coin(most of the time is used to promote an ICO).
Most of the time airdrops are the one-time task. Just do and submit.
BOunty program going parallel with ICO. We have to do some task week by week in order to get the stakes.
ANN Thread :   https://bitcointalk.org/index.php?topic=3293429.0   
Bounty Thread :   https://bitcointalk.org/index.php?topic=3359820.0   
if you are looking for the bounty program in good ICO, Guarium project is for you. visit above links to more details.