I wouldn't spend too much money...don't want em to be expecting shit all the time. Maybe she'll end up making bullshit like every week you have an anniversary type thing.
To be honest, Arkan has the best advice. Nothing he has said about women ever has been wrong. He knows what he's talking about, so just say fuck it and take her to McDonald's.
make her lunch/dinner and write her a nice letter. 6 months is a big deal if you are relationship noobs, but if youve both had experience with dating and romance, then keep it low key but classy. making dinner is a good plus, especially if its more advanced than macaroni with cheese and kool-aide. try pasta and seafood. dont buy her that bullshit flowers and candy, thats just like giving women a handshake. dont expect much from her, but after dinner go out somewhere and try your luck with the romance.