We went on a week-long vacation at the end of March.
Here's what I remember: traffic was fairly heavy around Kona and Hilo. Heading south out of Kona on hwy 11, the road is twisty and hilly, traffic thins out, and I think there were okay shoulders. Heading north out of Kona on hwy 19, the road is fairly straight, with a great shoulder (because they do the Ironman out of Kona), but the headwinds are going to kill you. As I recall, the great shoulder went all the way up 19 to 270, then it got kind of skinny going up 270.
You can ride 270 all the way north until it dead-ends at Pololu valley, then hike down the steep and often-muddy trail to the beach where a lot of people camp out. We saw two families setting up camp (tents and all) when we did the hike.
Heading south around the island, the weather will get more Oregon-like; Hilo was rainy and grayish, the volcano was 60s and showers (the visitor center is at 4000 feet, it's a bit of elevation), and the whole east-side of the island was steep and cooler than the west-side.
For snorkeling, you want to be on the west-side of the island. That's where all the warm weather is, and all the great snorkeling beaches.
You'll be doing your fair share of climbing and descending, that's for sure! But it would be a lot of fun.