You can either walk up and down the mountain or take the cable car to Niederwalddenkmal. We rode the cable car up (worth the scenic views from the semi open-cable car) and we chose to walk down to see the vineyards close up. I was happy with our choice, and it was a casual 1 hour hike down, I estimate.
A plaque at the top will explain the purpose of the monument which is to commemorate Germany's victory over France in the Franco-Prussian war of 1870-71. The bronze sculpted figure of Germania is impressive.
As an added bonus, you get a great view of Rudesheim and the river. I would definitely include this as a must-see for Rudesheim.