Drop to the ground in a conspicuous location during the talk and start speaking in tongues. Eat whipped cream right beforehand to make it look like you're foaming at the mouth. Then right before anyone touches you, sit up and say, "Praaaaaaaise the Lord! I knew he would be happy I came to this convention."
Dress like Santa Claus.
Wipe your shoes in manure and walk around a lot, then sit & stay during the session to gross people out.
Bring your karoake machine and start talking in it during the sessions.
Wear a Halloween wizard or witch costume and chant during the songs.
Wear a giant scary cross and kiss it all the time. Tape a picture of the pope inside your bible; take it out often and kiss that too.
Put a snotty tissue into all the donation boxes, or even better, put some petroleum jelly on a condom and donate it.
Ladies, come out of the restroom with a condom stuck to your hair. Be sure not to wear a wedding band. Then when someone brings it to your attention, start carrying on loudly so as to get maximum attention. Have some male friends nearby in suits pretend to be elders and forcibly remove you from the auditorium.
Sit way up in the nosebleed section of the auditorium and laugh each time they say something stupid and say "ooooooh, I'm scared" when they talk about the rules.
Take your significant other and make out.
Wear clogs with clunky heavy soles and let them "accidentally" fly off your foot when you walk down the aisle during the session. I did that (really) accidentally and it caused a big commotion.
Strike up a conversation with a stranger nearby during the break and say you're a virgin pregnant with the Christ child. Especially effective for male apostates. (You might get hauled off to the psych hospital for doing that though.)
Tell someone the book you bought at the convention yesterday danced around your hotel room, glowed, a ghostly smurf arose and screamed when you tried to burn it.