Hard to beat the location or the view! We only had one night to crash in the Hilo area and wished it could have been longer since the setup at Steve & Su's place is so great.
Our personal recs:
Relax at Liliuokalani Park. (You can get poké nearby at Suisan Fish Market or cold treats at Alii Ice Co Ltd). Walk out to the island. Read a little about it.
Mochi from Two Ladies Kitchen. (There will be a line. Wait in it.)
The Hilo Farmer's Market runs several days a week, but if you can be there on Saturday you'll probably have access to the most stuff. It's pretty fun.
The hike up the side of Rainbow Falls is closed as of Oct-2021, so don't plan to spend any more than about 10min there.
The drive up to the visitor's center on Mauna Kea is worth it, even if you can't go all the way to the top. If you happen to be there when the moon is gone or slim, you'll really get to see some great starry sky.
Akaka Falls has added remote pay stations and attendants to the small parking area. Of course, when it's full, most people just park along the road. But be prepared to have zero cell signal from Honomu, on.
Please don't forget to be on the lookout for nene in the road. And pack your trash out when you are hiking around. It's kind of ridiculous that they keep having to remind people about this stuff. Don't be those people.