Add some props!
If the theme of your map is medieval then add some medieval props!
I see you have a beach/sand area so add coral/water buckets/and stuff like that!
Snow theme, add snow piles/snowy trees/Christmas tree/candy canes/etc.
(ahem, my guide on how to make a Christmas tree and a Candy Cane: Christmas Themed Ideas)
Space theme, add props floating out of nowhere (check out this guide to make a broken/fallen spaceship: Help me make a spaceship (just a barrier art or with props))
Tell me if you need any other ideas!