Bring or mail your own decorations or goody basket(s) to the resort. You could also order something through Disney Florist but it is rather pricey. For decorations, you could hang up a string of lights, stockings, maybe a mistletoe. Some people order a cheap small (fake) tree and have it sent to their resort to sit on the table. For non-holiday decorations, you could create a banner to hang or window clings. Just make sure whatever you use to decorate does not damage the walls or windows (no tape, push pins, etc-- use suction cups, damage free wall clips, string to tie a banner to curtain rod), and make sure you remove everything before you leave. You could create luminaries. Take a colored paper lunch bag and cut a shape into each side- bonus points for Mickey heads- and put a battery operated tea light inside. Some tea lights will change colors and these look really cool with the lights off in the room. You could bring balloons and blow them up when you get there if you don't mind them not floating (my kids never did- they liked just hitting them around). You could get character balloons and I've even seen some packages of the character ones at Walmart that have a glowstick inside so they light up.
We ordered a basic cooler gift basket from the Florist for the whole family, then I packed character tote bags with various Disney goodies and left them out for the kids as a surprise from Mickey and the gang when we got to the resort. We also did Tinkerbell gifts where we left out a small gift each night for the kids to find in the morning (sort of like the Tooth Fairy). If you're going around Christmas, you could do this but have the goodies left in a stocking. Between the goody bags and Tinkerbell gifts, we left things like character suckers and gummies that I made with Disney molds, small stuffed animals, tsum tsums, tattoos, new character pajamas, bathing suits, beach balls, goggles for the pool, character sunglasses, pins, glow sticks, magic washcloths, autograph books-- stuff that they could use during the trip.
You could also create personalized invitations to restaurants or any other special things that you have planned for the trip and have them left out for the kids to find in the morning.
Our TA made the kids some really cute bags with mickey ear decals and their names (you could probably order something like this from Etsy or just get cute bags without their names) that we filled with goodies for the plane ride. Things like snacks, small character coloring books and character crayons, small books, etc to keep them occupied. Dollar stores usually have tons of Disney themed stuff. We surprised the kids with these right before we were leaving.
