There's plenty of good advice here on the cycling front, so I'd go for making it a really enjoyable occasion in other ways: plan to take a nice picnic, stop at a good pub on the way home with either a pre-prepared 'Thank you' card for joining you in doing something you like doing, or a homemade payback voucher that will entitle her to get you involved in something she likes doing. Don't forget constant feedback to her on how much you are enjoying being with her, her performance, etc. This obviously matters to you, so make sure she knows how much you value her effort.