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While it may be possible to exit Canada and re-enter without attempting to enter the United States, in your circumstances this would be inadvisable. Consider the situation where, for some reason, you were denied entry back into Canada. You would be stuck in between border crossings, with no possibility to go either way.
(I have done something similar, exited the US at the bridge in Buffalo and re-entered the US without entering Canada. However, I hold a Canadian passport so there was no possibility of getting stuck.)
If you are eligible for ESTA, I would suggest getting one before exiting Canada, so that you have somewhere to go if things go wrong. If you're uncomfortable about getting an ESTA and not actually using it, then enter the US after you exit Canada and spend a few hours there.