From your hotel, you could take a cab to Hung Hom and then take the KCR East Rail train to Luohu Border in one hour. Go through the border, you will enter into Shenzhen. Then, you could take the Shenzhen metro visit the Window of World(https://www.travelchinaguide.com/attraction/guangdong/shenzhen/window.htm), the Shenzhen Splendid China(https://www.travelchinaguide.com/attraction/guangdong/shenzhen/splendid.htm) and other spots around it. Finally, take the same way back to Hong Kong.