Is our purpose just to happily sit with God in heaven forever?

Thank you. The idea of heaven we have is what we know from the Bible. So we cannot know about everything that is going to happen there until we get there. Our primary purpose is to glorify God and to do his will. We were made for God and by Hm. So when we get to heaven, we will not be sitting simply. We will continue to glorify God through whatever we do there. Some of those things would include worship, singing and praising, serving etc. Heaven is definitely an exciting place to be. Do go through the resource by David Jeremiah on the topic of heaven.