I tend to use the Plumbo Workout Dance Video for my sims that can't afford fancy equipment or time to go to the gym if you happen to have. Dance one is good for just weight loss with the power sculpting for muscles.
As mentioned above lot of foods can contribute to a sim gaining weight quickly. Grand meals indefinitely make them gain lots of weight and sometimes you gotta watch out if you have sims in other hosueholds you aren't playing and they're going to food stalls constantly. Noticed this in city living few years back and had sims gain A LOT of weight. Not sure if it's been fixed as I haven't played in the main area in so long.
