In my opinion, bounties are a good way to earn cryptocurrency without actually having to spend money to purchase them. To do that, you participate in activities that help spread the word of the ICO, unlike in airdrops, where you get free coins simply by signing up. For bounties, you need to do things like: Add a signature to your profile and post often, create information on webpages, youtube, etc; tweet information about the cryptocurrency, partake in their Telegram, and the list goes on. You can expect to pick up coins by doing more tasks listed in the bounty program.
For example, I'm partaking in TravelerToken's ICO and they're offering 15,000,000 TLT coins over a couple of months. Since 1 ETH = 5,000 TLT, they are essentially allocating 3,000 ETH worth of coins to bounty hunters. A certain percentage of this bounty is in the signature campaign. I receive stakes each week for being a Member. Newbies cant partake, legendary members receive bigger stakes. Therefore, it'll be more worth it if you start posting here often to upgrade your member rank in order to receive a bigger stake each week (more coins). Each bounty will give you different amounts of coins and their value will differ.
For your reference, feel free to check out
TravelerToken's [ANN] thread. At the end of the day, picking a good bounty is key - so don't forget to do your own reading before participating! Good luck!