Ueno Zoo is the oldest zoo in Japan, and an easy way to spend at least half a day wandering around the surrounding areas and inside. Its just a short stroll away from Ueno train station.
The surrounding park area is nice for going for a short walk. Theres also tandem boats you can ride in around Shinobazuno Pond for a small fee.
Of course once you are inside, there are all of the animals for you to see. For a small fee of Y600, its really a cheap day out in Tokyo. Keep in mind that its not open on Mondays.
I’m not sure what the schedule is, but when we went, there was a food market going on. One of my favourite things to do in Japan is to eat the market food, especially fried chicken and yakisoba noodles. This was on a weekday, so maybe it caters to the tourists?