WordNet sense 1 (stimulating interest or thought):

thought-provoking

WordNet sense 2 (rousing or quickening activity or the senses):

stimulating

WordNet sense 3 (disturbingly provocative):

intriguing

WordNet sense 4 (serving or tending to provoke, excite, or stimulate; stimulating discussion or exciting controversy):

provocative

WordNet sense 5 (requiring full use of your abilities or resources):

ambitious

WordNet sense 6 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):

difficult, hard