Challenging the serve is what the returner should look for most often during the match. Aim to make the server uncomfortable with the first shot they see coming back over the net—using the server’s force against them.
Still thinking in terms of big targets and ample height over the net—without trying to end the point—the returner should looking to move the server into an uncomfortable position, so that the next shot back in play is easier to control.
An attack is more viable against second serves, of course. The goal shouldn’t be a return winner, a rare result (although that kind of play earns a pat on the back when successful). Instead, attacking the serve should be viewed as an opportunity to flip the script.