Note that the non-stop bus 629 usually departs from Admiralty MTR, not the Central Ferry. Only 5 departures from Central Ferry are from 9:35a to 10:55a. If you can't make those, you have to go over to Admiralty.

The Airport Express runs every 12 minutes and takes 24 minutes. It's a few minutes walk to the Central Ferry buses. 629 departs at 9:35a, 9:55a, 10:15a, 10:35a, and 10:55a. Takes about 25 minutes.

Coming back, you need extra time because 629 will only bring you back to Admiralty. You then take the MTR to Central, walk over to Hong Kong, and then take the Airport Express back. [AE ticket will cover the MTR transfer.]

Make sure you buy the same day return AE tickets. They cost the same as a one-way. HK$100 for adults.

MTR/AE info: http://www.mtr.com.hk
Bus info: http://www.nwstbus.com.hk