To make a public-speaking activity more 'appetizing', I've combined it with a banana-split incentive.
Give a short presentation on speaking (focusing on speaking in Church, specifically), giving tips on what to do and what not to do (such as don't say "um", be sure to make eye contact, etc).
After the presentation, give each girl a frozen chocolate-dipped banana in a bowl and a pen & paper, but tell them to wait to start eating because they can earn toppings for their 'banana split" by spotting the 'mistakes' in a faux talk that you will give. Give a short (1 minute) talk while really over-exaggerating mistakes that you have talked about (1 for each topping that you have to offer). Have the girls write down the mistakes you are making and they get an extra topping for each correct "mistake" they identify in your talk.
I got the idea for little banana splits from this cute dessert:
but you could use regular banana splits with ice cream, etc. You could assign each "mistake" they catch a specific topping, like....
no eye contact with the audience - cherry
saying 'um' - whipped cream
telling the audience that you didn't prepare - nuts
talking really fast - chocolate sauce
mumbling the name of Jesus Christ at the end - sprinkles
MATERIALS NEEDED: Prepared 1-minute "talk" for you to read, frozen bananas and toppings, paper & pens
PURPOSE: Teaching girls some of the skills needed to give a better talk in Sacrament meeting (and public speaking in general)
UPDATE: We did this activity last week and the girls had a lot of fun. I tried to emphasis some of the common 'mistakes' or 'less-effective' approaches that are frequently seen/heard in our area, such as....
I taught them that we do not discuss sacred things (such as sacred aspects of temple ordinances/teachings and Patriarchal blessings). We have a lot of people in our area who talk openly about their Patriarchal blessings in talks/lessons, so many people think this is acceptable to do in a church context, when they are meant to be shared only with immediate family members. Sacred is sacred. We shouldn't make exceptions because we're speaking in a Church context and develop a "well, we're all Church members here" attitude (especially because sometimes the congregation is not all Church members).
Another thing I discussed is that that we tend to hear (almost weekly) the same phrases over and over "I know without a shadow of a doubt...." "I love each and every one of you, even though I don't know all of you..." "This talk was mostly for me, because I learned more than anyone..." "I would be remiss if I did not stand and thank my Heavenly Father..." and "I didn't want to give a talk, but the Bishop cornered me in the hall..." I explained that while all of these phrases might be true for the person using them, saying those same exact phrases out of habit or because everyone else says them is pretty close to 'vain repetitions' and tends to come across as less than genuine. I encouraged them to try to avoid using phrases just out of habit and to think of their own way to say what they need to.