ATMs are widely available. Credit cards are accepted in most hotels and restaurants; some budget places only take cash.
Hotels A HK$10 or HK$20 note for the porter; gratuity for cleaning staff at your discretion.
Restaurants Most eateries, except very cheap places, impose a 10% to 15% service charge, but it is normal to still tip a little (under 5%) if you're happy with the experience. At budget joints, just rounding it off to the nearest HK$10 is fine.
Pubs and cafes Not expected unless table service is provided, then something under 5% of your bill.
Taxis Tips are never expected.