My cousin is turning 25 on November 25th and we have planned a Vegas
trip for 5 couples.  I'm looking for ideas on how we can celebrate his
birthday over there.  I want our entire group of 10 people to be a
part of it so it can't be anything for just guys.  Something like a
group activity but concentrated on him.

If everyone is healthy and fit, an "indoor skydiving" party would be a blast:

http://www.flyawayindoorskydiving.com/lv.html

Check these websites
http://www.adventurelasvegas.com/group_events.html
 (Information on Kayaking, Biking, Hiking, Horseback Riding, Guided tours)
http://www.vegas.com/attractions/freeattractions.html?f=m0at&t=freetxt
(Lists all the attractions like shows)
http://www.vegas.com/sports/recreation/?f=m0at&t=rectxt
(Lists all kinds of outdoor/sports adventures)
Here is the google serach I used. There are plenty of more search
results on ://www.google.com/search?hl=en&lr=&ie=UTF-8&q=group+activities+adventure+in+las+vegas

Hope this was helpful
Regards Have fun in Vegas

Thanks for the links.  The indoor sky-diving looks really interesting.

I tried to find a link to a good hot air balloon site in vegas, there
is no sensation like it in the world. There is definitely a 15
passenger hot air balloon that used to operate out of Vegas, it is 10
stories tall. Most balloon company's will have you help set up the
balloon, take you for an hour long ride, then feed you some light food
and champagne. They usually offer a momento like a pin or shirt etc.

If you can't find a 10 passenger balloon, go for two ballons from the
same company...work for a discount since you are taking two trips for
them. They aren't as booked solid as they would like you to believe.

Mother911-ga