Tickets for Mickey's Not So Scary Halloween Party are $38.95 for adults 10+ if you purchase them ahead of time. They're $45.95 if you purchase them that day (you can see the dates and buy tickets online at disneyworld.com).
I think it's worth it. My aunt, mom, and I are going on October 11...they have a great fireworks show and IMO, the best WDW parade ever. Getting "free" candy is always fun (even for adults

), and there's special entertainment, in addition to many attractions being open (hours are 7pm to midnight, but you can usually get in a little earlier, from what I'm told).
This is separate admission to the parks, so if you have a MYW ticket that does not include days at the park for every day of your trip (for example, you are in WDW for 8 days but you only have a 6-day hopper), it might be a good day to spend the afternoon at DTD or the BW and then head to MK for the party. I think teenagers would have a really good time (I know I would have loved it when I was that age - I'm 26 now).
I'm a little tired, but I'll think about cool birthday things to do and get back to you (I know in the meantime, many people will have great ideas for you!).
Have a great time on your trip (I'll be there 10/6-10/13 - Woo hoo!)!