Another vote for the 28-75, a must. Make sure you have a spare battery to hand as you will need the flash for virtually all shots.
An easy diffuser that works well is tissue paper and a rubber band. Fold the tissue paper and test, fold it again and test, fold it again and test. Carry enough tissue and some spare rubber bands in case it falls off / they break. Tissue works surprisingly well.
If shooting singles/couples then f4 is enough (it will be 1/180 for flash) , for larger groups then f5.6 (and 1/180 of course) is good. In each case try the tissue flash test at the approximate distance for singles /couples / groups so you know what ISO to set, start at 100, if not bright enough then 200 and so on up to 800. Write down the results of your testing on a card and put it in your pocket .. no guessing or getting it wrong later !
Also tell everyone to close their eyes on a count of 1 and everyone open their eyes on 3 .. so you say 1,2,3 OPEN - and fire ! No closed eyes - ever
Take two shots of each pose, just in case something happens/is in the background, that you didn't want.