There can be many things depending on what you are using the system for.

If you search Google you will get a lot sites with suggestions on this.It is not possible to advise some universal to do list, as it will get mostly opinion-based answers.

I am giving you some links to consider, but bear in mind that you don't have to do all things given in those lists. Some may be absolutely unnecesary for your particular situation. That's why always read and understand why this or that tweak is done.

http://itsfoss.com/things-to-do-after-installing-ubuntu-14-04/

http://www.noobslab.com/2014/04/thingstweaks-to-do-after-install-of.html

http://www.webupd8.org/2014/04/10-things-to-do-after-installing-ubuntu.html