Definatly try to get out of the terminal!

I'd second the walk along the corniche. There is also a small park at one point (if you walk away from the water and cross the road) about half the way around, which houses an art gallery. Although the gallery will be closed the design of the park area is nice - lots of water flowing along channels like swimming pools and nice lighting etc. nice enough to pass the time anyway.

If it's open, there is also an ice rink (made of plastic I think) in the City Centre mall - but at that time it's likily to be closed.

The Sheraton hotel is at the far end of the Corniche. If you walk all the way around it should be easy enough to get a taxi back to the airport from there or/and have a rest in the lobby.

Oh, and from my experience, the modern, clean and expensive looking taxis (turquoise and silver coloured) tend to be cheaper than the orange and white cheap and old looking taxis!