Find out her physical fitness level and take her on a day hike that doesn't challenge it. I like to bring a semi-gourmet spread and maybe a stove for making hot chocolate. If you know a good place, show her a pretty view. If you can do it during a season relatively free of biting bugs and check the weather forecast so it doesn't rain. Make it a date the first couple of times so she enjoys it, then see if she would consider a hardier adventure.