My husband and I spent our honeymoon on Oahu and Maui last year. This January we're fortunate enough to go back to Hawaii with frequent flyer miles and we are debating spending a few days on a new island in addition to Oahu. This will definitely be a budget trip- looking to spend 100 a night or less. We love to snorkel and explore during the day with a casual drink or two at night. Any recommendations on which island would be best for people on a budget with our interests? Is Kauai too rainy in January?(budget places to stay are welcome as well!)

if you dont mind not having tv and phone,
but want to be oceanfront, in a small hotel with friendly people, try Kona Tiki in Kailu-kona on the BI.
Cost about 80/per night. Can do day trips from there, although I recommend 1 or 2 nights near the volcano.
very basic website at www.konatiki.com
also, great snorkeling 5 min to Kahaluu beach, or 20 min south to captain cook. drinks downtown at Jamesons or north to fancy Waikoloa hotels.
(I'm flying to Kona in the AM for 3 weeks

Big Island doesn't have a lot of good snorkeling because so much of the coastline is lava flows. However, whereever there is enough of a beach to safely enter the water, the snorkeling was great! The big attractionon the Big Island is the Volcano, and it was truly amazing!

There are budget places to stay if you know where to look!
We stayed at the Country Goose B&B in Volcano Village for $75 a night. www.countrygoose.com Joan was a wonderful hostess, and our room had everything we needed and then some!
In Hilo, we stayed at the Wild Ginger Inn for about the same price. http://www.wildgingerinn.com/ It didn't include breakfast, but the bed was very comfortable, and it was nice to sleep to the music of the babbling creek behind us.

My mistake... Wild Ginger DID have a small breakfast, but we got on the road to early to enjoy it.
You MUST eat at Ken's House of Pancakes in Hilo! MMMMM!!!

I've read good things about Kona Tiki. Was the lack of AC a problem? I'm guessing it'd be fine as long as there is enough wind. Any other recommendations? We'll only have 4 days or so. Thoughts on Big Island vs. Kauai?

Big Island Snorkelling is some of the best anywhere! We prefer it for our trips since it is laid back, large, and more spread out than other islands. That being said after a few trips we like to venture to other islands for a few nights to break it up - Right now we are debating Kauai or Molokai for 3 nights after our 7 nigths on Hawaii...