[howox]

Abs: sit ups. Too easy? Hold something heavy against your chest. Too easy? Hold something heavier.

Fault! This is adding weight. Better luck next time. :)

[chrishynes]

Hanging leg raises are what you're looking for. Start with bent leg, then straight leg, then toes to bar etc.

[quickthrower2]

Pilates (wothout reformer) offers harder ab exercises without external weights