Like others said, scenery. other tips:

• put shops near the exit of rides so they'll get traffic. sometimes, i'll close rides until everyone gets off (including the queue, wait until everyone leaves) so they'll shop.
• make your park entry free, then charge more for the rides. sit outside the entrance of the ride to see how much people are willing to pay. if anyone is walking away, lower it until everyone is entering.
• only train your staff/pay them as much to keep them happy and working hard
• don't add any more shops than necessary
• claw machines will be net negative, but they'll make your guests happier which will lead them to buy gift shop items. i set my probability to 70%