A hostess gift could be just the thing to break the ice. You could choose something simple like a bouquet of flowers or a bottle of wine. Gourmet chocolates are also a nice touch.
Try to avoid going overboard, though -- you might look like a showoff and make your boyfriend's parents suspect that you're trying to buy their admiration. Of course, be sure to run whatever gift you choose by him. You wouldn't want to bring wine if his parents are teetotalers, or sweets if one of them is diabetic.
Afterward, remember send your boyfriend's parents a thank you card. It doesn't have to be anything elaborate, just express your gratitude for their hospitality and the opportunity to meet them. You might also mention something that you discussed to personalize the message even more.