Thank you all very much for the suggestions. You have expanded the options that I thought were possible. I'll probably use a combination of these techniques.

While I was searching for postcards I came across a site that describes how to design a postcard in an 'editorial' format and print them on "110lb fluorescent yellow cardstock" with a laser printer.

http://www.businessknowhow.com/marketing/postcard.htm

I have a laminator and I might laminate them for extra effect before putting a stamp on.

Thanks.