Rain does not have to ruin a Hong Kong trip. The city has extensive indoor spaces and excellent public transport, so you can often switch plans rather than cancel the day.
Museums
Keep one or two indoor museums as backup options.
Shopping centres
Large malls can provide food, toilets and air-conditioned shelter.
Transport
Use rail and covered walkways to reduce exposure between stops.
Shoes and clothing
Waterproof footwear or a compact umbrella can make a major difference.
Safety
Heavy rain can create slippery surfaces and localised flooding; follow official weather warnings.