valvetrain mods

I have a 2001 Celica GT, and i belive its DOHC but im not sure.
my question is what kind of mods can i make, while still being able to drve it to work on a daily basis

You can do almost anything, as long as you don't do it to an extreme. Almost all engine mods come in various degrees of change. The most radical ones are for off-road only. Less radical are ones that are street-legal but make the driveability very poor. The most mild ones keep all the good stuff and still give you a decent hp increase. What you get is up to you and your bank account.

thanks for the reply
so what your saying is the VVT-i will not be affected by any mods

Unless you are able to modify the program that controls the valves itself, probably not. YOu might be able to find someone or a company that can do that for you.

Since the VVT is computer controlled, you can't change it unless you change the ECM input (Ne - RPM signal from the distributor) or the computer. The way to modify valve characteristics is replace the cams without changing the VVT setup.