A little anticipation can make a decorating night feel bigger without adding much work.
Offer three or four choices
Keep the list short enough that younger children can understand it.
Vote before shopping
Once the theme is chosen, you know which decorations are actually worth buying.
Give both cakes a related twist
One can follow the theme closely while the second becomes the family's wildcard version.
Make the vote part of Tomte night
If The Tomte Cake is your activity, announce the theme a few days ahead so everyone arrives with ideas before the baking begins.