What maintenance needs to be done at 150,000 miles?

Nothing magically changes at that mileage. Keep on with regular oil changes and wheel alignments. There isn't much to a tune-up anymore. Have the spark plugs replaced on time. Most last up to 100,000 miles now so yours have probably been replaced once already.

Even cheap gas has lots of detergents and other additives already so running cleaning chemicals in the system isn't necessary.

The timing belt should not be overlooked. Many import engines are "interference" engines. They develop very expensive internal problems if the belt breaks. With "free-wheeling" engines you just coast to the side of the road in a puddle of tears when the belt breaks but repairs still take a few hours. If your timing belt hasn't been replaced already, consider yourself on borrowed time. That's one thing to not put off.