I love the fitness activities in the game. I don't mind if my Sim's gain weight, it's usually because I've been cooking a lot, because I really enjoy that part of the game and having the Sims sit around the table to eat all the food and desserts and chat. Or they might gain San Myshuno festival pounds when I make them eat all the food from the stalls, even if they are full, so they can learn the recipes. I sometimes give them some diet ice cream when I want them to learn recipes lol. They usually only go up one body size though and then back down again pretty quickly, because I like to play a fitness activity at least every few Sim days or once a Sim week and some of my Sims have a daily routine if it's part of their story.
The yoga mat is maybe my favorite. I like that it is safe for the elders to use and Sims can carry it in their inventory and use it anywhere. I also like jogging. I right click on their icons and then follow them around on their jogging paths to see the scenery and look for rocks to dig or plants to harvest. I wish they added "Go for a walk" alongside this. The climbing walls and Star Wars lightsabers are fun. I also really like renovating the Willow Creek gym and adding the yoga class, climbing machine, dance floor and a basketball court or swimming pool outside. If there is an extra room in the Sim's home, I usually add a little home gym with a punching bag, treadmill and weightlifting machine. I hope they add more objects one day like a stationary bike, small hand-held weights, a ballet bar, skipping rope and more workout routines on the TV fitness channel. In general, with only one fitness activity every few days, my Sims don't really change their shape much, but I heard there is a weight gain bug.