Coca-cola also has a nearly limitless budget to put commercials on the air, on the radio, in the air ports and anywhere else they darn well please. They've also been around for about 122 years. Their success did not come from guerrilla marketing, and it does not depend on it in any way. Sure, it probably helps - but they're a power house already.
Stickers strategically placed in the areas where people who care about what you're selling will see them. An intriguing image, a way to find more information, and a motivation to do so. That's what you need.
A business card is going to get you a very small return. Gimmicky or not - a lot of times those gimmicky ones get tossed because they're annoying. If you give me a metal business card, most likely I'm going to toss it in a drawer and never look at it, if I don't throw it away right away. Why? Because that would be annoying to have in my wallet, and it wouldn't fit right in holders and stuff. Kevin Mitnick's business card is a lock pick set - that I would keep because I pick locks. But if you just handed me a piece of metal all it's going to make me think is how annoying it is - is that the emotion you want associated with you?
Maybe I'm being pedantic but a coin isn't a business card. A business card is a card. A coin is a coin. And it falls into the same category as the metal business card - It's basically useless. If you had some kind of value attached to it - "Show the coin for a special show rate!" or if you managed to build some kind of prestige around being a fan and that's how you show it ... well, then it's got something. But the coin itself has no value and would probably just end up in a drawer somewhere. I've looked into getting custom coins made before - it'll probably run you around $300 for 100 units of a fairly basic design. No color. I would advise being sure you know how to make that investment worthwhile.
The first thing you need to do is define your product. What are you trying to market? Then you need to define your audience. Who is going to buy it? Then you need to figure out where you target market will be when they are thinking about purchasing your product. Then you have to figure out an eye-catching way to let them know that you can fulfill the need they are thinking of. At least - that's my thinking on the subject. I'm not an expert and don't want to come across as saying this is definitely true.
