I'm reading through some of the older questions (the really old dusty ones). What can be done to bring some of the hidden gems back to the surface? Any suggestions/guidelines/tips as to when to use edit/answer/bounty or other ways to contribute as I read?
If the question is good and has no correct answer, simply edit any part of it (including answers) or add a bounty and it will go to the top page (and RSS, if it's really old). But if it's a fully-resolved one and there's nothing to add to it, let it stay where it is because we have search here and SE posts rank well in search engines.
The front pages and "activity" sorting are for new questions and questions that require assistance. They aren't a "featured" section.
You pretty much got the three ways to "bump" a question. Take note that dnbrv is correct that many of our posts will remain in a high Google ranking as long as they're discoverable (keywords match, which tagging does wonders with).
If a question is missing valuable information to solve a problem (or a new way, or a way not previously mentioned), by all means add a new answer to the question. If it's just a minor problem in an existing answer, go ahead and edit the existing answer instead.
If the question is missing valuable information and you can't provide that information a bounty is a great way to get attention for the question and encourage others to answer. You can also just bounty a question "To draw attention" if you want to; that's accepted practice and built right into the system. If you really think a question is underrated, a bounty will help get it noticed.
Old posts often need some tidying up; spelling errors, dead links, poor formatting, chatty body copy. Always fix up "buried gems" when you see them so they're more presentable; you should do this with all questions though.
Worth an extra note are tags and titles; these help people find questions. If a question is missing an important tag add it. If the title is unclear, fix it. Titles should give a strong hint as to what the title is about. There's some people who title their questions like "Help designing a form". Instead edit it to something relevant like "Can a multi-part form have a submission button on each page?"