Hen do - bake off activity?

This sounds like a very thoughtful idea. I'd suggest keeping it reasonably accessible and flexible - maybe try doing something like heart shaped shortbread or ginger biscuits/ginger men? And then invite people to decorate them in teams or each do something personal and your sister can guess who did which one. They can then be pretty/silly (you could have 16 groom biscuits for example) etc. That also means that you could pre-make some basic dough on standby in the fridge case timings go awry on the day and/or make it when people arrive depending on timings. You can put a batch or two in the oven, they do not take too long to bake so there is not a big delay, you can all have a nice drink and a chat whilst they cool down, and then have some fun decorating together - with biscuits everyone will have something to do. You can also flavour a basic dough easily (teams could flavour their dough with choc chip, vanilla or lemon essence etc depending on everyone's tastes). It also means if someone isn't too keen to join in (maybe check for allergies/special diets) they won't feel exposed and still be part of the activity whilst friends get decorating? It's also not too expensive to buy for and biscuits are easy to transport. Also, a tray of misshapen ginger men if it all goes wrong will be a giggle...but if you all try to spend hours making something expensive and elaborate and it goes wrong/people are not enthused then it might feel a bit frustrating! Good luck planning! x