If you purchase small pickling or garden cucumbers, be sure they are not wax-coated. It impedes the pickling process.
Beets come in various colors -- magenta, yellow, or white. For canning, be sure to choose smooth, firm beets with the taproots attached. The taproots are the long main root upon which beets grow.
Knock on watermelon to check for its readiness. If it sounds hollow, then it's ready to use. Try pickled watermelon, which goes great on any relish tray or with Greek yogurt for a healthy breakfast.
Summer and winter squash should have hard rinds and stems that appear fresh. The skin should be glossy and brightly colored, without any cuts or bruises. Remember: the smaller the squash, the better its flavor and texture.