The most common Bounty tasks:

Signature Campaign: You have to insert a code signature that in your bitcointalk profile. The number of stakes depends on your ranking.
Facebook and Twitter: You will get stakes if you post, repost or retweet from the official account of the project.
A blog post or video: If you are a big media influencer, you can create content for the project.
Translation Campaign: If you have language translation skills then you can professionally translate content for the projects. This is usually one of the highest paid tasks.