Here is my $1 brain-enhancing, neuron-pruning, stimulating* baby mobile. The $1 was for the bubble blower ring that's the hub of the mobile. The rest is stuff we already had in the house.
*In truth I haven't checked, so I don't know if my baby's neurons are actually being pruned. But she's 7 weeks old, and will wake up in the middle of the night and entertain herself watching the mobile, and you can't ask for much more than that!