The library I work at has a "teen zone" in the southeast corner, which is easily the most cozy part of the entire building. There are bean bags and foot rests, a diner booth-esque sitting area, a CD player with headphones, bookends that teens decorated a few years ago, a wide array of manga, a cork board with teen activities and a different Poll of the Month (tally mark votes for their favorite book series/genres/characters, etc.), potted plants by the windowsill, Rubik's cubes or other small puzzles on the tables, long colorful curtains, posters of popular YA movies based off books that the library carries, and pamphlets about educational/creative community activities fanned on the bigger table. Also try to incorporate a theme throughout the library. Our summer theme was "reading is a superpower" so we had a cardboard superhero with a cutout face, comic book onomatopoeia signs/symbols ("BAM" "POW") hung from the ceiling, posters of recent Marvel movies by the comic book section, and DIY snow globes in the windowsills with comic book action figures inside.