31 synonyms for challenging such as invite, arduous, bid, and dare. Challenging is an adjective, and its antonyms are easy and uninspiring.
Arduous, burdensome, demanding, difficult, energizing, exacting, exciting, exigent, formidable, gruelling, hard, heavy, inspirational, inspiring, laborious, onerous, searching, stiff, stimulating, strenuous, stretching, taxing, testing, and tough.
Bid, dare, defy someone to do something, invite, summon, and throw down the gauntlet to.