As an expansion, Sims 3: Generations adds a wide range of content to help expand the Sims experience through each of the life stages. It's an incredibly detailed addition that makes the entire experience more realistic and amusing.
From the beginning of life till the end, Generations has content for every Sim. Starting in infancy, Generations gives each baby the chance of receiving an Imaginary Friend doll in the mail from an Uncle's magic toy factory. If the player keeps the toy and allows it to be played with enough as the baby grows to a toddler and then a child, they may be gifted with an incredible surprise. The toy can actually morph into a walking, talking Imaginary Friend that only the child can see. They can interact with the child, run errands for them and will become their best buddy in the world. The Imaginary friend is a bit attached to the child, making for a few minor annoyances when sending the child to use the bathroom, but otherwise, they can be absolute best friends forever.
Each Imaginary Friend has their own personality traits that influence them, and they are essentially non controllable Sims. And there are in game ways of turning your Imaginary Friend into a real person, allowing them to be controlled and treated as a normal Sim for the rest of their lifespan. They can even grow up to marry the child that they were friends with, having children who are half Imaginary Friend and who have a doll form that they can turn into.
Children also have even more things they can have fun with as children. There is a costume chest that allows them to dress up and play dinosaur, astronaut or royalty. And even without the costume chest, they can 'hold royal court', dressing up in a cape and crown and presiding over their court which is in reality a simple chair. I'm not entirely sure why all of my children had the exact same hairstyle/crown when switching to play royal court, but the giggle I got from watching them play pretend was more than worth having to change clothes later. Children can also play pranks on people, setting booby traps around the house such as whoopee cushions and shower dye. And occasionally they'll ask to invite friends over for a slumber party where they can play with the new water slide.
The pranks continue into teenage hood, expanding into pranks that can be pulled at school and pranks that can be played at neighbors' houses, such as lighting flaming bags of poo on their doorsteps. Teenagers can also have high school proms and learn to drive from their parents, an amusing experience to watch as they slam on the brakes driving around the neighborhood. Sometimes parents will even get a free trip offered to them, allowing teenagers to throw parties when mom and dad are away.
And both teens and children can go away to one of five different Boarding schools, which completely removes the sim from player control. However, they finish their education and come home with multiple skills, depending on the type of school they go to. Be careful though; one of my sim's children called their mother in the middle of the night, begging to be allowed to come home from the prep school because she hated it there. There are also afterschool activities that kids can get involved in, meaning that they stay at school a little longer having fun. They can get badges to display on their walls, or have performance recitals that the whole family can go see. They even get to go on field trips at school; one to the police station earned my kids a mounted police badge to put on the wall.
For adults, there are bachelor and bachelorette parties that can be thrown before they get married, as well as new places to whoo hoo once they have: the shower, and the brand new Tree House object that children can play in. Those activities obviously can't go on at the same time. And older Sims can even experience a Mid Life Crisis, events that have been lampooned countless times in media and that are usually just as hilarious when they happen to the Sims, but that are occasionally very annoying.
Not every sim is going to have one when they age transition, but when they do, you can either take them to get therapy to end the experience early, or you can try to fulfill some of the wants to give them a good moodlet when the experience is over. The crisis lasts for several days. Once a wish has been promised, it can't be cancelled and must either be fulfilled or the crisis must end for it to go away. I had one sim in rapid succession wish to Move, Quit Job, Flirt with someone besides his wife, and Divorce his Wife. Not a very nice fella. Another sim just wanted to get a makeover and a tattoo, in addition to a few other minor wants. It's all very random what your sim will get and you just have to realize that you have to be careful promising mid-life crisis wishes unless you're willing to complete them or pay to end the crisis early. It is very amusing though to watch your sim obsess over wrinkles in the mirror.
And teenagers have their own mini crisis that they can experience in the form of a mood swing. Like a mid-life crisis, they can suddenly get the desire to do all sorts of questionable things. One sim I played wanted to skip school and pull a prank at the school as part of his mood swing. Guess he wasn't too fond of his public school, huh?
The memory system returns from Sims 2, only it's been upgraded into a scrapbook with pictures. Sims will get good and bad memories of certain events in their lives and players can actually share those memories with other Sim players online if they want to brag or lament the trials their sims are going through. There are several new objects in the game that are fun as well, such as the long awaited bunk bed, and the Chemistry station which makes a comeback, allowing Sims to make different potions for use. Teen sims and up can go on dates again, getting positive memories from the experience of having good dates with people.
Daycare is added in as a profession for those who also have Ambitions, allowing sims to care for other children in their home for money. Some of the new traits, like Nurturing, and new rewards such as Super Nanny help make the profession easier. Other rewards, like the Clone Voucher are purely for fun, as they allow a player to make a clone of their favorite Sim, letting them play that sim from childhood up into adulthood all over again. And for players who like having a dynamic neighborhood but who may have one particular sim they'd like to keep around, there is an Age Freeze reward that can be bought that allows that sim to stay around forever, unless you use the Birthday Cake to age them. And as an interesting little add on, Mysterious Mr. Gnome can actually have an entire gnome family now, filling your yard up with his children. And in Create a Sim there is now the option of adding body hair to older male sims.
The entire expansion simply adds to the complexity of the game, helping to fill out the Sims personalities and the experience of playing them. The new interactions are amusing and fun. There really isn't much that can be complained about with this expansion except for the care that needs to be taken when actually deciding to select Mid Life Crisis wishes. Beyond that, Sims 3 Generations is one of the must have expansion packs (along with Ambitions) for enhancing the base game experience.