1) What if they said: Just bring yourself!
If the host protests that you needn't bring anything, you can still knock their socks off and show up with flowers, a bottle of wine, or some dessert. Caution: if you bring dessert, bring candy or something single-serve and pre-wrapped. The host may have already accounted for dessert and you don't want to outshine their baking or make more dirty dishes.
2) What if they are allergic to traditional gifts?
Keep allergies in mind as well. Are they allergic to flowers? Bring fresh herbs from your garden. Are they off sugar? Bring cheese. Lactose intollerant? Bring candy but be sure to read the label to avoid dairy products.
Candy Cane Marshmallows!
3) What if there are restrictions related to their home?
Also consider the state of their house. Do they live in rented housing that doesn't allow for candles (a dorm, I guess)? You can get flameless candles, scented room oils, or a bag of gourmet pretzels. Are they pack-rats who don't need one more thing? Bring consumables and eat them while you're there or just help with the dishes after the meal :)
Bring a note. Either write it beforehand or during the party while they're distracted and leave it in their mailbox on the way out, on the bathroom sink before you leave (I've done this one!), or somewhere conspicuous. They'll love the double thank you that they get - verbally and written.
Of course not all hosts need/want/expect a gift. Chances are when they said that they are happy to have you - with or without the bottle of wine - they aren't lying. Be on time, wipe your feet and compliment the chef. Just being a good guest will get you invited back!
