My youngest daughter recently had her 4th birthday and wanted a Frozen birthday party. Ironically her sister had one when she turned 4 a couple years ago.  Here are some fun thing we did-
You can see pictures and information on the last Frozen party we did here.  It also has some free printables.

For the decorations I purchased a LOT of balloons and made some balloon towers.  They weren’t too hard to make, just take a little bit of time.  We didn’t use a weight or pole for them so they did fall over if we weren’t careful, but as long as you didn’t touch them they’d stay where they were supposed to.  I made a balloon arch with helium balloons to go across the top and that actually helped both towers stay in place better.

To make the balloon towers:
Enlist a LOT of help blowing up balloons, or have a pump.  Start by tying balloons in pairs.  Then twist one pair around another pair until you have 4- that is a layer.
You’ll want to tie some ribbon around the middle of the bottom layer, and if you want to add a weight this is where you’ll want it. 
Then stack a second layer of 4 balloons on top and use the ribbon to wrap around 2 of the balloons once or twice then pull the ribbon.  That will pull the 2 layers together and hold them in place.
If you want a pattern with colors etc. like I did making it look like diagonal stripes, you just alternate the direction of the layers.

The backdrop pictures I purchased at Walmart for $4. It came with the big ones we used by the balloons, as well as the “Happy Birthday” banner and 2 smaller ones we put over by the food table.