As a Dubai resident I cannot recommend Wadi Adventure enough. The facilities are excellent, it is great value for money (AED 100 for a 2 hour Kayaking lesson, and AED 100 for an hour or so's white water rafting) great fun and a really good active day out. Unlike Dubai's chaotic Atlantis water park, this place is well organised, with tip top food and great teachers on hand to make sure you get the most out of your trip.
I recommend leaving Dubai early (7am) and getting there for 9am because the park is quiet then and you will need a full day to do the activities and also relax around the lovely swimming pool.
I went with two girlfriends and we had such a laugh. Make sure you book your activities about a week before as they tend to get booked up.