Lillian, you seem "well traveled" to me, why don't you have an "International Party", everyone comes dressed in apparel, from someplace they've visited over the year.
You could continue the theme, by serving foods from all these different places.
If you decide to go this route, don't forget to invite me, I love trying new & different things.
Oh I had another idea. If your meaning of "dress up" is formal, you could do a "Years Gone By" theme. Then everyone could wear formal dresses, etc... they already have hanging in their closets. You could even make a game out of it "What Year am I"

Be kinder than necessary, for everyone you meet is fighting some kind of battle.