Somewhere around eighteen months our son discovered the word no, and used it for everything. Shoes, breakfast, the bath, the park.
What helped
Offer two choices. Not "shall we put shoes on?" but "red shoes or blue shoes?" Both answers get you out of the door.
Give a warning. "Two more slides, then home." It does not always work, but it works more often than surprise.
Pick your battles. A raincoat on a sunny day hurts nobody.
What to remember
Saying no is how toddlers learn they are separate people. It is a stage, and it passes. Mostly.