The low budget solution would be several plastic bottles in the day pack I’ve had for years (yet rarely used). But fishing bottles out of the pack whenever I want a drink is inconvenient, and having them flop and rattle around while walking/climbing is annoying.
Or I could go with a dedicated hydration pack. That would mean extra cost. And I’d want to carry other things besides water, like maps, camera, emergency gear, clothing layers, and food.