Consider software not hardware

You can't really purchase any additional hardware within this price range that will maximize your experience other than the excellent and inexpensive 50mm f1.8 prime lens that can be picked up for around $100 or less used. You could use that for some detail shots with a background bokeh, etc. The CPL filter was also mentioned in the comments above and since you have a tripod and a shutter release you might also want to consider a neutral density filter.

However, given the listed equipment I would encourage you to use try using your tripod, shutter release and the 18-55mm lens to do some HDR shots. Old buildings against dramatic skies (which HDR will further enhance) look particularly interesting. To do this you might need to invest into some software such as the NIK HDR Effex Pro or similar to process the exposures into an HDR image. There is all sorts of software and plugin filters for various image processing applications that will allow you to be creative with your photos. I already mentioned NIK and I admit I enjoy using their plugins so I would also recommended the Color Effex Pro or the Silver Effex Pro for Black and white images. The plugins are more then your budget but still reasonable and they will surely make a bigger impact on your images then any $100 piece of hardware.