There are lots of things you can do in the Sims 3. My friend and I came up with this list a while ago and are trying to play through every one of them (we've only completed like 4 lol):
-Legacy/Uglacy
-Immortal Dynasty
-Make a household of 100% randomized Sims. Then play through several generations with 100% randomized Sims.
-Make a household of 8 Sims, with no trait being repeated between any of the members. (This is possible, lol! My friend has done it. Without using pointless traits like never-nude and vehicle enthusiast, too)
-Make a household of Sims with contradictory traits and see the madness unfold.
-Challenges (ex. Baby Boomer, Asylum)
-Create a story and post it on this forum!
-Collect everything, get highest quality cuts, fish, produce, make perfect meals, etc.
-Create the most dysfunctional family ever
-Trap all neighborhood Sims in a basement with no way to get out
-Make a male Sim and make him have hundreds of babies with the townies. Ask one son to move in. Have the son impregnate everyone, etc. Make these men be the most hideous men in the world.
-Create a female Sim and see how many babies you can have before you die. (For extra challenge, try to have a different father for each child and be a single mother) make sure to meet all milestones
-Create a household with 6 toddlers and two young adults. Try to play through legitimately and accomplish all milestones (Potty training, walking, talking, getting an A in school, max as many skills as a teen, get a part time job. For added challenge, accomplish lifetime wish as a teen
-Live your life as a homeless person
-Make a young adult female, or two young adults who dislike children, are mean spirited, etc. and make them have tons of kids.
-Live off the land. Be completely homeless.
-Make a person and try to have them complete every Lifetime Wish.
-Remove all houses/Sims in towns and create a new one
-Max All Skills with one Sim
-Max All Careers with one Sim
-Have your Sims life revolve solely around their wishes. If they wish something, you make them do it.
-Make a maze for your Sim(s) to get through
-Become a vampire and then make everyone else become a vampire.
-Detonate everything
-Create a family with an elderly man/woman and 7 toddlers.
-Buy every lifetime reward with one Sim (for added challenge, start as young adult or adult)
-Make a movie