I have a mobile attached to the top of the cot which I use to occupy 14 wk old DS while i go off to the loo etc; but for sleep he sleeps with his feet at the bottom of the cot so the mobile is behind him - i think he'd be stimulated by it but admittedly i haven't tried it.

He started having a cuddle toy (Tigger head with soft fabric/no body) at bedtime from 12 weeks which worked surprisingly well to soothe him, I tuck it under his arm and he immediately pulls it to his mouth and starts sucking on Tigger's ears - but i put away it to the far corner of the cot once he's asleep..