If you are willing to spend the time and money preparing well in advance, you can host an elaborate Hogwarts Celebration, but of course, you can still have a fabulous Harry Potter party without breaking the bank! Select the ideas you like the best that fit your budget and what time you have available to prepare, then go to it!
Even small touches can go a long way toward enhancing your environment to be more Potteresque For any decoration display, clustering your props together makes more of an impact. If you only have one or two props placed around a "normal" room, your guests might miss something really clever, but if you cluster those same few props in one place, you will have a "Potter-themed area" that, even if small, will make more of an impact your guests will definitely notice. Even if you do have a lot of props, still tailor your decoration displays to the scale of the space you do have. You still have to have room for your guests to have fun!
A small end table can have some books added and maybe an example spell drawn out on a scroll or on a black paper "chalkboard" to become a Transfiguration classroom.
Another small table can use a globe light fixture cover turned upside-down on a 3-wick candle-holder with a teacup and saucer over a fringed scarf "tablecloth" to be Divination class. Add a copy of "Unfogging the Future" or other Divination books if you like.
A wall shelf can have decorations added like books, wands and feathers for a Charms class.
Get out your spookiest skulls, gargoyles and other Halloween items to set up a Defense Against the Dark Arts class.
If you have very limited space to decorate, concentrate on your food or activities table by using a "magical" glittery tablecloth, some spell books, wands, small owls, and other magical props around your food plates and drinks or activity supplies.
Herbology class is easy since that can be with your fruit and veggies to serve to your guests. Add some cheap small exotic-looking houseplants or check craft stores for floral pick sales for artifical plants to place around the edges to make it look more magical than just mundane munchies.
Potions class is also easy since that can be your drinks area. You can find interesting bottles at thriftstores, import stores, discount stores, and even just by paying attention to what your friends are going to recycle. There are many interesting bottles around today! To make your display more magical, buy some inexpensive bottle corks at a kitchen supply store to stopper your bottles instead of the original screw-top or bottlecap lids. You can be as elaborate or as small-scale as you need to be for a mix-your-own-potion drinks area.
If you have a hallway mirror anyway, make an extra sign that attaches around it using the Mirror of Erised description.
You can hang portraits on hallway doors to give the impression that the doors are "hidden" and might require a password or spell to gain entry. You can paint or draw these portraits yourself, you can purchase poster-size prints of classic paintings if you like, or check thriftstores for old paintings that already include the frames.
Small craft decoration owls can be placed around your decorated space. If you have enough, you can "hide" them and have an "ogle the owls" contest for your guests, giving a prize for who finds the most owls! Larger and more realistic owls (that are also more expensive) can be found at garden stores since they are used to keep other birds away from gardens.
We have plenty more decoration ideas collected here, even to create clever specific props, so please browse the entire list.