You can do something that
- it's interesting to you, something you will be passionate about. Something that is fun to program.
or
- something that will help you learn new things, that will help you grow as a programmer
or
- something that solves you problems (for example open source library that solves some problems you encounter when programming on mobile platforms)
or:
- something that solves problems of other people
> Please advice what type of side project I should work on.
"should" is Wrong word. You can. It's more like mental attitude. Take a look around on hacker and startup communities. Many things arose from sideprojects.
But even if you don't have this kind of sideproject mental attitude it's still good to spend some time programming on your own, at least to get familiar with other tools/libraries and approaches than you have in your work. If you can really don't know what to do it's still better to make anything just for learn new things (for example to learn popular framework you don't know yet)