Don't worry about what other people think. They don't know the awesome powers of the cube!
As far as what you are thinking about doing, there are so many different ways you can go. The underdash lighting you can either get the OEM interior accent lighting or you could do your own DIY and get your own LED light strip and tap it into another light.
Stereo system... Well I would just get whatever you want. I am not big on Audio, but you can't really get any worse than the stock two speakers....
Wheels! OMG Wheels! Heh, there are so many wheels out there, but unfortunately Nissan gave the cube and 4x114.3 pattern which makes it that much harder to find some nicer wheels. BUT, If you are ever planning on lowering your cube I think that should be the first thing before wheels. Just two inches does wonders on the looks and the handling with the available clearance still there.
What color cube did you get?