1. Play in age groups. Have a group of kids, group of teens, group of elders, etc. Use clubs to have those groups do things age appropriate together, including your elders. Age everyone up together, so they always have their friend group throughout life (except for elders who will die off at their own rate). If you're using premades to start, choose a neighborhood and go in and age all the non-grownups to the same age (toddler, child, or teen) so that they all grow up together as a group (siblings can be twins).
2. Make places that the game doesn't have a setup for. Be creative. Like make a teen center or after-school center for kids and have the group go there, or just set it so it's off limits to everyone but the group and maybe an "employee" like the cool kids counselor or something. If you have more teens than will fit in a group, you can make 2 teen groups and give them both access. Or make offices for your adults to go work at. You can even set up cubicles with half walls. I guarantee you that if you bring a group of people to a place full of computers, they will all immediately "get to work," haha. Or at least it will look like they're working while they play games or whatever. Have a manager's office that is locked to everyone but the manager. It's especially fun to have an office in a highrise because it makes for some great screenshots. You can also do this with residential lots. Right now, I'm playing a Victorian era game and I have a factory where everyone works (using woodworking tables), but it's actually a residence, so they also all sleep there in dorm-style rooms. Use your imagination to set up different venues and use groups and door locks to control it.
3. Set up entire worlds based on a theme. Delete all the premade sims and any buildings that don't match your theme. You can play an era, like the 70s, or the 1800s, or even the future. It's challenging but also fun to try to find all the right cc and to populate your world with the right types of buildings, clutter, people, hairstyles, etc. Just make sure you don't try to build the entire world first before playing any sims because it will take forever. Do it in stages and build a little more on with each generation.
4. Use reward trait cheats and also additional traits that you can download. The sims' personalities are lacking, so I pad them out as much as I can to make things more realistic and choose reward traits that I think would fit their personality.
5. Don't make your sims perfect. If every sim is beautiful and nice, that gets boring. For personality, go to one of the sites that let you randomly role personality traits and then just put in whatever you rolled. Or pick your own traits but always do 2 positive and 1 negative. For looks, I like to generate 2 random sims, then use them to create a teen child (so I can see what he/she will look like), then delete the random sims and change the age of the teen to whatever I need. While I'm randomly generating the teen, I stop at the one who looks the most realistic, rather than the one who looks the most beautiful (unless I happen to roll a raving beauty or something, which is rare).