Thank you all for the suggestions. After looking at the weather and doing some more research, I will likely postpone Hokkaido until next time, probably in the summer. April just seems like a very transitionary period when the snow is gone, the greenery isn't there yet, and the nature will be fairly barren. Instead, I decided to concentrate on northern Tohoku and catch the end of the sakura season. So, something like:

• April 22: Shinkansen from Tokyo to Aomori, arriving around 2 pm. Visit Aouimi Park, Nebuta Warasse, eat at a restaurant with shamisen performance or the izakaya with the Nebuta masks out front.
• April 23: Go to Utou Shrine and A-Factory. Early lunch, then train to Hirosaki to see cherry blossoms at Hirosaki Park. Maybe visit the Apple Park. Dinner somewhere, then see nighttime illumination of cherry blossoms at the park. Train back to Aomori.
• ​​​​​​​April 24: Nokkedon breakfast at the market. Either Asamushi Onsen or Mt Hakkoda and an onsen there - need help here.
• April 25: Back to Aomori. Switch to Resort Shirakami train to Akita and enjoy the ride. Spend night at Akita.
• ​​​​​​​April 26: Train to Kakunodate. See weeping cherry blossoms, Samurai district and its houses, visit the woodcraft museum. Train to Sendai. Dinner somewhere.
• ​​​​​​​April 27: Check out of the hotel, train to Matshushima, boat tour of the bay, Zuiganji Temple, eat lunch. Then Back to Sendai and take shinkansen to Tokyo. If the weather isn't great for boats, may skip Matsushima and go straight to Tokyo.

Any suggestions are welcome.