Take a look on Youtube with the search terms "bicycle generator". There are a few slick ideas and some right down to redneck level. Many utilize a treadmill DC motor as they are more effecient but there are some using an alternator with good results also.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock