It is even possible to have a breakfast here, though you can go up without breakfast, just see the opening hours on the official website https://www.vjuimenergiebunker.de/.
The photo shows the tables outside, but there are also tables inside, so you can come in the cold season.
Getting there: parking lot near the bunker 53.510028, 9.990528. By public transportation, find the route here https://www.hvv.de/en, the address is Neuhöfer Straße 17, Hamburg.