Just keeping watching the anime and don't bother with the manga until you've caught all the way up and are really eager to see what happens next. Manga people are probably going to give me shit for saying that, but I think that's the best way for you and your gf to experience One Piece together. Blowing through manga chapters quickly and quietly would not be nearly as fun.
Also, although Water 7/Enies Lobby is probably my favorite arc, it's not the best in a "it's all downhill from here" sense. There's still a ton of really cool things in store and, depending on your opinions, you and your gf might find them to be better than Water 7/Enies Lobby. I can think of 2 arcs in particular that I personally think are on par with it: Sabaody Archipelago and....well, it might be best not to mention the name of the other one so you two can go in completely fresh. You'll know it when you get there.
Make sure to use a filler guide though.