Are you going to fill the jar with liquid? If so, I would go with an "Expanding Brain". I know it seems silly, but these super-compressed sponges really expand huge if you leave them in warm water for a while.
One expanding brain will fill a jar in about a day -- and look incredible. I love these things, and the're afordable.
i have a brain for you , my sister's boyfriend as one he doesnt use so that would be perfect ......
.............sorry when i saw the thread title i just couldnt resist
joke asside the color changing brain like frankie's girl mentioned is very cool i have one in my mad lab , the batteries usually work all evening too ....
or the ones at dollar stores that grow in water do a great job (at least the ones i found here do ),espacially for the texture if you have kids touch it i have 5 or 6 in a uge water jar for the past 2 years and so far so good .
I actually used a whole cauliflower in a jar with colored water one year...add a splash of milk to make the water sort of murky and it doesn't look half bad. Smelled really bad after a few days when I took the lid off the jar to throw it away, though.
I laughed at your thread title too, and thought "Don't we all...."