The morning is for Lantau Island. From the subway you quickly reach the cable car and go up to Ngong Ping with the "crystal" option - the experience is worth it. Tickets can be bought on the spot; most of the time the queues are small or non-existent. Up there, time flows differently. Tian Tan Buddha imposes peace, and Wisdom Path is a short walk, 10–15 minutes, which perfectly complements the visit. In total, 3–4 hours are enough so that you don't feel rushed.
When you get off, a surprisingly pleasant outlet mall awaits you below, if you feel like shopping. Then change the register completely: take the subway to Kowloon. The promenade in Tsim Sha Tsui, Avenue of Stars and, if you are with children, the Hong Kong Science Museum. If your legs can handle it, go up Nathan Road: shops, malls, food courts where you can eat without stress.
In the evening, if it's dark, head to Temple Street Night Market, getting off at Yau Ma Tei. At 20:00 PM, return to the promenade for the Symphony of Lights. The crossing back to the island with the Star Ferry is part of the show. Alternatively, you can do the opposite: at 19:55 PM, take the ferry from the island and see the lights directly from the water. Here, the show is so touristy. The buildings are beautifully lit, messages about Hong Kong appear. It's worth checking out, but you can also miss it.