You can go to the afternoon tea in shorts or whatever you would wear onboard during the day, just no swimsuits. Just don't make the mistake I made when I went to one, I totally spoiled my appetite for dinner, was it worth it, YES!
Slides are a good option as already mentioned. Are these daytime capris or night-time capris? My sister wore fabulous silk capris with something close to "hooker shoes" on one night and turned every head on the ship. (She would turn heads in a sweat suit, so maybe that's important?)
There are two kinds of afternoon tea ... a formalish dress-up sort of thing on some cruise lines and just tea-type food on others around 4 PM.
The daily cruise ship newspaper will give you all the clues you need about what to wear.
Don't worry too much about wardrobe. You will see anything and everything. Wear what feels right for you.