When you're sightseeing, Victoria Peak in Hong Kong has beautiful views of Hong Kong island and Kowloon.
The lines at the Shenzhen Bay border crossing always seem to be shorter than the Lok Ma Chau or Lo Wu HK/mainland border crossings, but unless it's a major travel day you can probably get through any of these in 30 minutes or less. If it's a major travel day it might take 2-3 hours to get through customs and immigration.
Also, the Google Maps views between Satellite and Maps view are intentionally off by a random amount (sometimes up to a half kilometer) due to a Chinese law. However if you go to
ditu.google.com the maps view will be the same between Satellite and Maps view. The reason this version works fine is because this maps view shows the Chinese territorial borders as claimed by China.
Edit: you can leave the lights and AC on in your hotel room 24/7 by sticking any random card into the slot by the door. Otherwise the AC will turn off when you leave the room.