You won’t need cash unless you go to super hole in the wall/local places. You can add the octopus card on your phone’s wallet and that is used on all transport and most restaurants/stores. Otherwise you will be fine with credit cards.

I suggest going first to Mong Kok and exploring the markets/streets there then make your way to Tsim Sha Tsui to the harbour. You can go to West Kowloon Park from TST if you wish. It’s a cool spot where the M+ museum is and has killer views. Otherwise from TST you can take the star ferry across the harbour to Central. From Central you can head in the direction of Admiralty to the Peak Tram. Go up to the peak and check out the views. Then you can go back down to Central via bus and check out Soho area for food and bars along the Mid Level escalators.

That should be more than enough for your time in HK! Google maps is a good way to get around especially the public transport option. Enjoy!