When thinking about bruising, you need to both consider the implement and the location. I find that fleshy areas bruise much less easily than less fleshy areas and more concentrated impact leads to more bruising than more diffuse impact.
This cane usually brings out bruising on my thighs http://www.lovehoney.co.uk/product.cfm?p=15828&reviewid=152985#customer_reviews but it can take a while to come up because it's deep bruising and I have to be hit pretty hard. I find thicker, denser canes are better for bruising (there is loads of info online about density of materials) but the best thing I've had used on me is a metal baton.
I have got some pretty impressive bruises from a heavy belt doubled up, but I think it was more the force than the implement. Pretty much anything can bruise you if you get hit hard enough.