Have you thought about doing some spring planting? Maybe it's too girly for boys, so, sorry, but you could get some flower pots, paint them red and then they boys can decorate them with LM stickers or Cars stickers or whatever. Then they can plant some seeds in them...I mean it does involve dirt so that's gotta be somewhat intriguing right? ;) I went to an Easter party last year where girls and boys did planting and it was pretty fun.
Depending upon how many kids will be there you could draw some sort of race track on the driveway with sidewalk chalk and have the kids run races. Maybe you could get some sort of mask or hat that is Lightening-ish and they can wear those and be part of the favor.
This is hard one...I'm sure you've done a lot already. What about a pinata? I'm sure they make a LM one!! It would be fun and take a while for sure. Since it's close to Easter maybe you could do a spin on an egg hunt. Presuming that you have a bunch of LM matchbox type cars, as well as other Cars charcters or cars in general, you could hide them around the yard and give the kids a chance to hunt them. The kid who has the most wins a prize. Or you could just put out one LM car and a bunch of other random cars and the person who actually finds LM gets something different.
Okay, that's about all I've got! Have fun!!