I currently use Git to backup my things in DropBox but I haven't found this easy enough. I am not going to create cron hacks. Does there exist some built-in way for DropBox to do backups?
Install one of the many services similar to DropBox (Google Drive client comes to mind).
1a. As an alternative to (1), you could try setting the dropbox folder an the google drive folders to the same folder. This might work perfectly but it also sounds like it might be a tad more unpredictable.
Regarding the initial feature request, dropbox is NOT for creating a second copy of your stuff on your computer. The whole point of DropBox is to put the second copy of your data on their servers (the data is versioned going back a number of days, which is kind of cool). In fact, if dropbox DID let you make a second copy on your computer, it would undercut the entire business model of dropbox where you NEED to access their service in order to get your data if it is ever lost.
If you must have the second copy on your computer, there are many solutions, I recommend SyncBackPro for its staggering list of features and ability to automate everything.