Have you tried the weight loss icecream? Sims are dropping weight like mad and get a happy mood buff from it. I completely forgot about this, easy mode for my lazy sims who don't want to work out.
You can also gain weight with weight of the world. Twin roses makes your sim flirty and they can catch on fire eating dragons breath. I love expermenting with all the moodlets and effects the machine gives. Taste of diet is my sims favourite though. I do have one sim who refuses to eat it and is quite cuddly ( to put it nicely) even though his wife and teens are addicted to it.