He says they worship Mercury, Hercules and Mars. These are most likely Odin, Thor and Tyr, respectively.
He says some of the Suevi worship Isis. Isis is probably Freya, or Frigga, or both.
What Tacitus does say, which is interesting, is that the Germans do not build temples to their gods, but worship them in sacred groves and natural settings. To give Caesar the benefit of the doubt, perhaps he saw Germans doing rituals in in the woods, and assumed they were simply honoring natural elements rather than gods per se. Who knows.