CHDK does not run on Canon DSLRs like the EOS 550D/T2i, so that option is out.

For extra features, the main firmware project to look at is Magic Lantern. Community reports mention it as a work-in-progress for the 550D, especially useful for video features such as manual audio control and audio meters.

For remote shutter release, a simple IR remote is an easy solution. It can trigger the shutter immediately or with a 2-second delay, which is handy for self-portraits and avoids improvised wiring.

If you want a fun DIY creative mod, try custom bokeh shapes. You place a cut-out shape over the front of the lens to change the appearance of out-of-focus highlights. It works best with faster lenses around f/2, but can also be used with the 18-55mm kit lens if the cut-out is made smaller.

So in short: no CHDK, Magic Lantern is the notable firmware hack, an IR remote is the practical trigger option, and custom bokeh is a simple creative DIY mod.