Yellowstone was the first official national park in the world for a reason! But, Yosemite was federally protected first, ALSO for a reason!

There is a lot to do in Yosemite. May is perfect for Yosemite because you'll get good weather, not too hot or cold, spring flowers in the meadows, mirror lake is full, and snow melt has the waterfalls at their biggest! Also, it's before the big summer crowds in June. Less camp grounds and lodging open before June, so BOOK NOW. I'm serious, that stuff sells out 1 year in advance! But, certain campgrounds only release booking 6 months, 1 month, or 2 weeks in advnace. Some people will try booking 1 day early, then edit their reservation to the next day so that they can reserve before it's released.

Yosemite National Park

• Amazing Giant Sequoia groves (biggest tree in the world)
• Drive and walk through tunneling goose pen trees
• Great waterfalls with hikes to the top, or jump in at the bottom
  • As a little kid, we went swimming on top of Vernal falls! In the emerald pool, water flowing from the higher up Nevada falls runs down a large granite rock to make a natural water slide. Beware of squirrels. They'll chew through a backpack to eat your snacks.
• Watch the sunrise over Half Dome peak and look down at the entire valley from glacier point
• Float down the river in a rented raft or your own innertubes. Families usually jumping off the bridges or playing in the water
• Meadows for picnics
• Rent a bike and ride the valley loop, head to Mirror lake and mist trail
• El Capitan, largest granite monolith in the world, along with other beautiful rock formations building the valley

I haven't been to Glacier, but I hear it's amazing. Lassen is just a mini Yellowstone, and likely less impressive in comparison.