A general answer appropriate to all kinds of jobs is to find activities that improve your working environment, or add quality to your project, or improve your skills. Your project lead or manager should be able to give you guidance as to what would be most useful.
If you're on your own to find useful things and need some ideas, here are some in no particular order:
- clean out a file cabinet
- purge data from your hard drive
- update the software you use regularly and have been putting off getting the new version
- improve your presentation skills
- read a book or watch a video or take an online course on a skill you could use at work
A few additional ideas specific to software developers:
- find code katas of interest and practice (http://codekata.com, http://codekatas.org, http://katas.softwarecraftsmanship.org/, or search "code kata" for others)
- make (or ask for) a code branch that's safe to use to fix technical debt (I'm assuming you're in a code freeze and can't work in your primary source)
- learn at least a little of a new language or tool
- find the keyboard shortcuts and/or macros in your favorite editing tool(s) and practice using them
One practice that may help you build a backlog of this kind of thing for yourself is to make notes while you're in the middle of a hot work cycle. Ideas you have about "this would be easier if only..." are good to put on a list, and when you have downtime, research the "if only...". Is there a tool that would do it? Is there a practice you can learn that would help? Find out when work is slow, and you will have something new in your toolbox for next time.