You might consider caffeine or theophylline (I would guess around 1 mM). Another possibility might be forskolin which directly activates adenylate cyclase although I have no idea of the concentration or its half-life in medium. Just out of curiosity why are you trying to avoid IBMX? I have a fairly potent adipogenesis medium recipe I use that has some additions to the typical cocktail out there if you are having induction issues.