Others have given some responses, but to address some of your questions:
1) A maintenance contract is basically a monthly/yearly charge for specified work. Real ones are probably as well thought-out as a prenuptial agreement. If your client sells products, you might cover 50 updates per month in your maintenance contract, and other such services.
My personal experience with maintenance contracts is hardware related. For gobs of money per year, the vendor will show up within 2 hours and get busy. Replacement parts will arrive within 6 hours.
2) "Reasonable" rates vary with the level of work. Check out the competition and what they charge.
3) Generating income is a matter of creating a menu. You've got the basics, but someone who's really driven and creative (that should be you) needs to come up with the rest. Someone already suggested you find out what your customers want.
4) You could start reading through the Web Design and E-Commerce forums for ideas, and start browsing Amazon for small business books.
Back to my #3 suggestion, really, if you're not very good with the business end, you've got a lot of catching up to do. If you're really in a hurry, find a partner who can help. Without this knowledge, you're like 50,000 other web designers out there trying to make a buck.