What is a good gift for my boyfriend for his birthday?

My boyfriend and I have been together for 6 months and I want to give him something really great and special for his birthday. Hes a huge metal head but i cant think of anything to get him. He already has the cds he wants and I want to do something personal. I was thinking of getting him a cross necklace(hes also really into God) and engrave it in the back so he knows he always has me and God with him. But for my birthday he painted me a beautiful picture and has something else homemade he still is going to give me so I kind of want to do more too plus I cant find a good cheap store that sells a good necklace like that. Any advice/help?