Well, the first question I would ask if I were in your position is "Is the couple registered somewhere?" If they are, then you can find a gift on their registry that they are sure to love because they've picked it out already. I'm assuming that they may not be registered anywhere because I would think your co-worker would have directed you to it when you were asking what to get, but you can check some popular registries here, just in case:
www.gifts.com/registry/wedding
Assuming they aren't registered, you can find a nice gift (the accepted equation to figure out monetary investment on your part is your budget times your level of affection for the bride or groom or their families) at a number of stores. One of my favorite off-registry gifts I got was a set of crystal champagne flutes from my next-door neighbor. Another I got was a beautiful silver photo frame which I used to frame one of my wedding photos. I've seen other creative gifts, like monogrammed guest towels, sporting tickets (for sports enthusiasts), and a subscription to wine or beer or chocolates-of-the-month programs.