i think that's a good question.
i remember having the same thought when i picked up my wife's phone to 'do up'
anyways you need to first check out all the various gingerbread/ICS roms in the development section.
see if you are happy with stock or would like a little 'difference'
do note that compared to stock there are always chances of a few bugs which you definitely can live with.
next or alternately i would suggest you check out all the emulators & games out there.
it's a play, so that's probably the first thing you would want to do
am guessing you new to this, so do a LOT of reading in this section, the Q&A section and the dev section... am sure you'll soon find more than enough ways to get the most off your phone.