That's a tricky one, I know! But fear not, I have some ideas that will surely make their hearts melt! Firstly, a thoughtful gift like a personalized photo album of you and your girlfriend would be a great way to show them how much they mean to you both. Or, if they're into cooking or gardening, maybe a set of nice kitchen utensils or gardening tools would make a lovely surprise. Alternatively, if they're the more adventurous type, you could get them tickets to a concert or show that they'd enjoy.