If you are going to visit the Museum in Mainz, save your money and not buy a Mainz card. Most of the attractions for Mainz is concentrated around the Markatplaz. You should be able to walk easily to these attractions.
When you stay in Mainz you should visit the nearby small Rhine river villages Oppenheim and Nierstein! Here you will find romantic small houses and wine cellars where you can taste the local Riesling in a very high quality. TOP of the TOP will be a stay in Nierstein in the first week of June every year. They celebrate there a local wine festival at the "Rote Hang area!"