That's very common for babies at age 15 months+, not only that they have their own mind to decision what they love to do, but also that they are very curious about the world and once they hit the bed, they thought they are going to lost their time to explore.
Just relax and let them to decide what they want to do. If they feel tired and exhausted, they will let you that it's time for bed, you don't really need to force them.