For me, it's all about keeping a routine. On a certain day each week, take your sims to an art gallery, a bar, a gym,or anywhere that has points of interest for them. The repetition can help you turn this into a habit, and getting out there will give your sims chances to meet sims with similar interests.
I have a community center for this purpose; tons of activities for different sims. Every night, one of my sims meets up with her girl friends and they do yoga together. Their kids tag along and play in one of the play areas, or watch TV, or paint.
