I think that submitting a patch or solving a bug will be helpful to get more involved during the community bonding phase. If that is best thing to do, Can you please recommend me something to work on ? (I submitted the patches [1] & [2]. But, all my submissions were small patches and I think working on a medium one will be beneficial during the community bonding phase.)
If you think doing something else right now is better, please let me know what to do.
Definitely lean toward what your mentor would prefer, it may be there are specific tasks/bugs/etc that would be related to your primary GSOC plan and build familiarity in the right area. But absent a particular direction, searching the help and bug list for 'missing function' and 'matlab compatibility' are usually easy places to start. There are enough of them out there.
Being a little self serving: