I think if they were once hippies and ultra-liberal then you lucked out. They're no "Meet the Parents" stereotype. They sound like they're nice, down-to-earth people who have no social prejudices and are willing to give you the benefit of the doubt. Meaning, they most certainly will NOT be asking you questions like "Why do you think you're right for my daughter?". Instead, don't bring up politics but rather SHOW them you're in the same mindset (equal rights for all, etc...). Just be a nice guy.
I recommend talking about music. They were once rockin' hippies. Let the conversation flow naturally though and don't be too much of bullshitter that your girlfriend won't want to see you anymore. Also, from past experience, parents really like it if you can bond with her little brother. Depending on his age of course. If he's 5ish and you make him laugh, you're golden.
Good Luck!
