I used to skateboard before I hurt my knee, and there are 3 spots that i can remember 1- corniche there's the 1 ramp there 2- it's a park close to Landmark i think it's called Dahl Al Hamam Park and they hand a sweet half ramp and a Rail also 3- In City Center, The arcade there's got a mini skate park, it's pretty cool. (the first 2 are free but the 3rd one i can't remember the charge, sorry)

P.s. there are a few groups of Bmx-ers and boarders that get together, try posting another question on here asking to join, you might get a reply (I'm pretty sure a few of them are on here)

Hope That Helped,
N.