
This isn't an easy thing for either of you to face; and I'm sorry you have to go through this. It's good of you to be there for her.
You have a chance now to say things to her that you might have been holding back, or feel too "corny" to say. Things that people who lose their loved one's regret having not said. Take full advantage of that; share with her how much she means to you and why. What would you say if you were writing her eulogy? She needs to hear what you'd write while she's still here on earth.
I'm assuming this mixed tape is part of the process of telling her how you feel; but you didn't mention the type of message you wanted to send. If you want the tape to tell her that you love her; then put love songs on there (like, "I will always love you" by Whitney Houston).
Songs by Louis Armstrong may be good.
Also a good song "God Must Have Spent (A Little More Time)" by NSync.
Return To Innocence by Enigma
I'm sorry if my suggestions don't fit. I'll be honest: I'm having trouble with this post because I wouldn't make a tape. I'd invite her over for lunch (or whatever works for the situation) and tell her how I feel and how much she means to me. We'd reminicse and have "remember when" conversations. I'd spend some time with her. I hope I've been able to give you some suggestions for your tape; and maybe some suggestions overall.
I'm truly sorry that you're losing someone important to you. If you need to talk, you can PM me anytime.