2,000 yuan should not include round-trip transportation to Hong Kong, or only the subway/bus from Shenzhen to Hong Kong.
Look for some small hotels/guesthouses, the price is 200-400 per night. If you don’t mind, you can also find a youth hostel. This can save some money, but you must pay attention to the safety of yourself and your belongings.
Tickets are not available, but you can go selectively. There are often discounts for booking tickets online in advance, which also reduces queue time.
In terms of food, it is recommended to buy bread and dry food, and drink boiled water for breakfast and lunch. You can find a place to buy lunch boxes for dinner, and try to make it less than 50 yuan a day.
For transportation, take the bus or subway as much as possible. For some routes, the subway is cheaper and faster than the bus. You can download the official ride-hailing app from the Hong Kong Transport Department's website and find a place with free Internet access to find routes.
Attractions you can consider include Stanley, Repulse Bay, Sai Kung, Lamma Island, Cheung Chau, the Big Buddha on Lantau Island and Tai O Fishing Village. There is no charge for these attractions, only transportation fees are required.
You can also go to Victoria Peak. You can take bus No. 15 from the Central Pier and go there directly. You don’t need to enter the wax museum or take the cable car. It doesn’t cost much.