Actually the citigate is a bit upscale for me but having gotten a good deal last July we spent 4 nights there. The fact that they have a free shuttle to the hotel to/from the airport also makes it very handy.

A. get an octopus card - makes traveling on the MRT and buses much simpler. There is a desk in the airport lobby or the Tung Chung MRT entrance. Deposit is refundable when you leave or keep it for your next trip (up to 3 years)
Note the MRT from the airport is some ridiculous fare (yes there are packages - still too much) but Tung Chung to Central on HKG Island is 18.50 HKD. And that stop is next to the central bus terminal
B. Take the tram to the Peak (on HKG Island) have lunch at the top. As well take the Star Ferry between HKG Island and Kowloon.
C. Take the cable car to the Big Buddha and the bus back (or the other way round). Or stop in Mui Wo and get the fery from there to Central
D. Take a ferry trip to one of the islands other than Lantau. Watch the shipping traffic.
E. If shopping is your goal there is a new massive mall as part of the Kowloon MRT stop. The multi mall complex at Ocean Road (near the star ferry) is equally huge.

Wander

Fred