As a cycling enthusiast, you cannot _not_ bring at least one bike to Toronto. As an introduction to cycling in Toronto, start just north of Yonge-and-Bloor (where street numbering zeroes) and ride, say, to Edwards Gardens (https://goo.gl/maps/Y3KYQ3DoZCbBAqCv6). You'll be amazed at how quickly you'll leave a concrete jungle into a forest.
As for the theft problem: it varies. If you’re with cyclists away from downtown, many leave competition-level bikes unlocked. But if you’re commuting and will leave a bike for hours downtown, then you must: