I am replacing my timing belt ( 2006 Honda Odyssey). Since this is likely high labour cost what else should I ask the mechanic to replace?
You absolutely should have the water pump, tensioner, and idler bearings replaced at the same time as you have the timing belt replaced. Everything needed comes in a kit and costs a bit more than replacing just the belt alone. This is standard maintenance on most every engine you replace the belt on, because if the water pump goes bad between belt replacement, it requires almost the same labor cost as does replacing the belt by itself. You may also consider changing out the thermostat and coolant at the same time, but is less likely needed. If you do have the coolant changed out, ensure they use silicate/borate free coolant (Honda Type 2 or equivalent). Honda engines do not play well with other types of coolant as it tends to deteriorate some of the gaskets in the engine.
If you are wondering how the belt is replaced and what is involved, I wrote a how-to article for Acurazine.com on replacing the timing belt covering the J32, J35 (your engine), and J37 Honda engines. While I know you aren't planning on doing the work yourself, it can give you an idea of what is involved to get it done, and possibly why it's easier to get the water pump done while you're getting the timing belt changed.
The water pump is also accessible underneath the timing cover on most engines. These are usually replaced together and sometimes included in the same service kit along with timing tentioners/guides.