FYI, Cal train does not go to Levi's Stadium. You have to switch to VTA in Mountain View if coming from the North. From San Jose, I'm assuming you'd just take VTA. VTA trains are small, and it's a major pain to get one after the game basically a huge slightly organized line. If linking with CalTrain, they might not run a special train, so keep in mind the last northbound train leaves Mountain View at 1051pm... which could get tight with a 6pm kickoff and a ~4 hour game. Very poorly located stadium for transit.
The hotels right next to the stadium will be major $$$ if not sold out, but if you can snag something within ~ 2 miles, it's not a bad walk, and it's not supposed to rain that evening.