how to fill a sketchbook page

The crowdedness is what makes it fun! At least to me! My advice is either learn to draw large scaled or add certain things to your sketchbook! I add stickers or doodles or even glue things onto the page. It can even be things as random as old receipts! Just let your creativity spark!

What I like to do for empty/extra space is either A) use sticker, B) put a candy wrapper there, or C) cut out a piece of paper and make a "door"

I usually just stick stuff like receipts, notes, tea packets, wrappers and other papers with different texture in mine. If it's a page dedicated to a character, you can add stickers or drawings of things that you think aligns with them, write song lyrics or make your own stickers by drawing on another paper, cutting it out and taping it there.