I would not put 18" rims on it. Mini's handle best on 16" rims. Also, since it is your sister's car, I would mod it based on here style, not yours. I don't know what she is into, but I wouldn't do anything she wouldn't like.
I know my sister would prefer some tweety bird eye decals under the hood (the headlight are built into the hood so there is a space for eye shaped decals, google it), instead of tinted windows. Also, if you can track one down on eBay the, "Mini Unofficial Owner's Manual," printed by Mini, is great for a few laughs.