If you are using B&W film, why not also try coloured filters to darken blue sky etc? A good brand of filter, with proper anti flare coating (e.g. Hoya) should not degrade the image at all and the effects can be very interesting. My canonet GIII puts the meter light sensor under the filter, so it should compensate for the filter factor by itself (unless you use graduated filters), although it is well worth calibrating properly (the compensation factor will be printed on the packaging) as I find that mine tends to underexpose slightly with a red filter unless I bias the meter. You should need 48mm thread filters. Most won't cost much, so get the best. It is false economy to buy cheap ones, which might not be optically perfect. I keep a UV filter on mine all the time to protect the lens.

David.