Susie,
The two options I am aware of for a convertible wanting a bike rack. The most obvious is to get the BMW model which reportedly is very well made but it kind of pricey and requires a couple holes through the plastic outer skin of the bumper. I believe if you order it with the car you might get covers the same color as the car but if you order it as an accessory it will have black covers but I am not sure. If it matters I would check. Installation involes removing the bumper cover so it seems a bit involved. Not too much is you are handy with tools but maybe a bit intimidating for some people.
If you look in the DIY section and search for "trailer hitch" you should find the thread about installing a hitch on a 1. BMW does not offer one in North America but an aftermarket supplier does. I don't think it is Hidden Hitch, it may be Curtis. You can also just google trailer hitch to find suppliers and find one offering a hitch for a 135i. When I have looked they have not listed the 128i but in the thread on the DIY here there is a report of somebody who installed it successfully. Should cost less than $150 and installation requires you to remove the bumper skin but you don't have to make holes in it. In addition, you need a bike rack that will slide into a trailer hitch. There are many models some of which will carry more bikes than BMWs.
You probably need to search here a bit more to see some pictures of how the final product will look both ways. I know there is a good thread on the BMW bike rack and on the hitch installation and result. I am not sure if there are pictures of the hitch with a rack in it but there probably are.
http://www.1addicts.com/forums/showt...=trailer+hitch
Jim