The Scavenger hunt isn't available anymore, but the best way to get more space is to refer your friends.
Refer Your Friends – up to 16GB
Referring your friends to Dropbox is how you can get a majority of the free space Dropbox offers. For every friend you refer you’ll get an additional 500MB of space – up to 16GB total. That means you need to refer 32 friends to get all 16GB of free space.