I'd recommend a tripod, honestly. If you're getting image shake that can help, and then you can stop down all you want to improve image quality. Or saving up for the 11-16mm or 10-122mm (for architecture) and fixing distortion in post. Or getting the 30mm f1.4 Sigma (or 28mm f1.8 Canon) as a general purpose lens, though it is not great for architecture.
The $12 fotodiox macro tube is nice for macro, though. That's one thing that's cheap and useful.
I agree that a tripod is what is needed here. Before I had my 17mm TS I often shot buildings and other architectural shots with regular lenses. If you have photoshop or photoshop elements, corrections are easy to do, just be sure that you have enough space around the building (becase as you are bending the picture, it 'cuts' away either at the top sides or the bottom sides). Another thing you could try in photoshop is stitching. If the focal lenght is limiting you, try shooting several shots of the object horizontally (with 25% or so overlapping to allow the software to find the information it needs) and let photoshop do the stitching. In my opinion that works well