hitchhiking up to volcano should be pretty easy. people there are friendly and as long as you don’t look crazy you should be able to get a ride.
getting a ride up to the top of mauna kea will be a bit trickier. not many cars actually go up to the summit because you “need” 4×4 and most just use the saddle road as a short cut to kona.
if you are looking to hike/backpack, i recommend mauna loa instead. in fact if you plan to do any backpacking i have a lot of experience doing that in hawaii so feel free to send me an email drakew@gmail.com
best of luck!

you might be able to sign up with some tour buses that go upto volcano if you don’t want to hitch-hike. i doubt there is anything like that up mauna kea upto the observatory, though.

hilo is wonderful on foot. and when i lived there, i didn’t have a car, so i walked everywhere. you can walk to mamo street in wednesdays and saturdays (early!) and go shopping at the farmer’s market. if she is still there, there is a woman on the right corner of the market facing the ocean who always has beautifully delicious tangerines. they are heavenly. i would buy a bag for 2 dollars and then walk down to the dock and chill out with the fisherman there. you can also walk to coconut island. it’s a little island near the big hotel on banyan tree drive. you walk through this beautiful japanese garden and there is a bridge to go across to the island. it’s very pretty. there are a few beaches, black sand or very rocky. some are loud because of all the kids, but it’s lovely. you can walk to the mall, which isn’t spectacular. almost everything in hilo, though is accessible by foot. you can walk to all the good food. i reccommend ken’s and ayuthaya. ken’s is near the airport and ayuthaya is on kino’ole. you can look it up in a phone book. ken’s is like a diner that serves everything. and ayuthaya is the best thai ever. check out the local food joints as you walk around. verna’s is a hawaiian gem. if you go downtown there are tons of lovely shops, homemade ice cream, food, clothes…. just walk. and the more you walk… the more things you’ll find.