Showcase meaning the glass case at the entrance? How fun! Is it specifically Halloween? Depending on your school system (and the religious and cultural differences of your student population) you might choose to tone it down a bit (no witches, vampires, etc) or you might go all out!...
Anyway, here are some of my favorites...
Fall/pumpkin patches (leaves, favorite fall books, scarves; if you have shelves you could put some sort of 3-D crafts students have made)
Skeletons (x-rays, "bones," books about bones/skeletal system, maybe have a black background with glow-in-the-dark stuff) <--- Kids LOVE this!!! I purchased some x-rays from
Smilemakers that I like to put up in the windows... but you might be able to go to a local hospital and get some for free.
There are these books called "I'll Read to You, You Read to Me" and I love their "spooky" halloween edition. (Sorry, I keep going back to books... I love children's books!). Anyway, you could post several of the poems from the book (or any halloween poems, really) in different sections and have a few "characters" (paperdolls cart is sooo cute for this!) and maybe stuffed animals around to decorate. The poems are NOT scary at all and appropriate for all age levels.
And I've never done this- but another idea would be to make it all candy oriented... cut outs of different types of candy, have candy wrappers, a few fun facts about candy (how much chocolate is produced each year, etc.), and yes, there are (of course) books about candy.
Good luck with this! I'd LOVE to see what you end up doing!!!