I just unearthed an old collection that I kind of forgot about - my soda bottle cap collection. All are pre-1970 and are cork lined. You can view the collection of more than 100 caps by clicking here. I love some of the cool fonts and neat graphics.
You know bottle cap collecting is a super fun collecting idea for kids. For the most part you can find them cheap at antique stores and flea markets. Search ebay for 'em and you'll see you can buy scads of 'em for pocket change with a few exceptions for the rare caps and ones with snazzy graphics. I would think kids would have fun saving caps from modern soda bottles too. There seems to be a lot of brands on the market.