The correct sentence is: "Sightseeing is best done either by a tour bus or by a bicycle." This sentence contains the appropriate use of articles for the nouns "tour bus" and "bicycle." When referring to an unspecified item, we use the indefinite article "a" before singular countable nouns. In this case, both "tour bus" and "bicycle" are singular and countable nouns, so they require the use of the indefinite article.
