you could have them bring their own pumpkin and do the carving there at the party ... call it a pumpkin carving party. We had this with friends BEFORE halloween and had some come dressed in costumes if they wanted..... have some inexpensive carving tools there with stencils (cheap ones are at garden ridge, dollar stores, big lots, etc.).... and then do the judging there.
Little prizes for winners: Bath & Body Works hand soaps (usually on sale for 3 for $9-10; or 5/$15.) or dollar store Candles. Or $5-$10 starbucks gift cards.
there's something about having an activity to do at a party -- it helps break the ice and open up conversations more.
I would put on the flyer/invitation -- that it's a potluck so everyone can bring one dish/appetizer/dessert to help with the event.
think about things for the kids to do ... like sidewalk chalk, sidewalk art projects. jump rope. draw hopscotch on the sidewalks. have some soccerballs sitting out. and some baseball/baseball mits there so some people can throw the ball around.
Good for you that you're doing a block party!! Kudos!! People rarely do these anymore and so therefore, we don't know our neighbors as well as we should.