There is a wonderful Hotel Vetruše (booking.com) in the city, where we spent the night and which we recommend to you. It looks like a real castle and is located on a mountain. In the Middle Ages there really was a castle on this place, but the current structure was built in 1897.
In addition to being able to climb the tower at the hotel, there is a wonderful observation deck nearby. Usti nad Labem is an industrial town, but I liked the view. Although, in other weather, the town would probably look very depressing.
From the hotel there is a fairly long hiking route through the mountains. We only walked to the first viewpoint Humboldtova vyhlídka, a map of the hiking trails can be found here https://www.openstreetmap.org/#map=17/50.65232/14.03884. From there you can see Hrad Strekov castle in the distance, which has controlled the way along the Elbe since the 14th century. If you go even farther, the view is likely to be better.
Also, you can take a cable car down to the city from the hotel. The lower station is right in the mall, which is convenient.
There is absolutely nothing to do in the city, its historic center was not rebuilt after the war. One of the few surviving buildings is the Church of the Assumption of the Virgin Mary.
From the hotel we saw a large rock behind the river (Mariánská skála) and decided to go there as well. We parked the car here 50.662542, 14.045797 and then walked to the cliff 50.662003, 14.051864. It took about 20 minutes to walk.
The place is abandoned, but the view is beautiful. In the distance you can see the hotel on the mountain.
Getting there: there is a train station in the city center.