I mean, I think the IT house is pretty much a no brainer at this point. You have the Creepy house he stayed in and the sewer. Just make it so you are walking through the house and end up in the sewer with Pennywise doing his thing. I think you need to have all of the forms he takes in the movie, especially the Flute Lady Painting, The Leper, The headless kid int he Library etc. Need to have a Henry Bowers appearance somehow, maybe him disposing of his sleeping dad? Having someone dressed as the flute lady eating Stan's face would be awesome to see too.
One scene I would love to see them do is Pennywise dancing in the sewer. The scare actor may get a little tired having to dance constantly, but I would love to see them bring that to life. Shouldn't be that hard IMO.