My ideas for activities so far:

Make smores in the fire pit (they are old enough).

Set up our tents in the living room.

Lots of glow sticks.

Capture the flag.

Complications: We have a smallish yard and my wife is out of town!

Make smores!
(Use a grill if you can't build a fire - or get one of those backyard firepit metal contraptions)

Great idea, LarryC.

Roast weenies (they're not hot dogs when you camp - they're weenies)

Tell ghost stories around your fire pit.

Rent a projector and show a movie outside on a white sheet (not something you usually do camping but it would be so fun) or rent a camping themed movie to watch inside - The Parent Trap from the 60s is a good one, and there's a Berenstain Bears one as well, and others that would be good for their age group.

Give everyone flashlights and let them hunt for stuff you've hidden in the backyard (a treasure hunt).

Singalongs and ghost stories are pretty square, huh?

Stage a "bear attack" in the middle of the party (dress up in a bear suit and chase all the kids outside, where some other activity awaits).

Once they're outside, give them a conch shell and tell them to figure out who the leader is, then send them on a "pig hunt" where they slaughter a piñata.

I'm going to a party store today to get party supplies for my nephew, who's having a party tomorrow for his birthday, which was yesterday. The party'll be at a nature center, and I think most of the activities are being taken care of by the staff there, which makes things easier on us.

My brother told me, at most all the kids' birthday parties he's been to with my nephew, they don't open the presents, saving them for later. I guess this is some ploy to spare kids' feelings or cut down mess or save time or hassle or something, but what the fuck? I used to get so excited to see the birthday boy or girl open presents (and of course I'd love opening my own). Sure, if these kids think it's normal to open presents in private, they don't know any different, but the same goes for those smiling starving kids you see in refugee camps or favelas; just because they're happy because they know no other life doesn't mean that it's a very good way to grow up.

Maybe I overstate my case, but is this non-opening of presents something they do elsewhere, too?

Guides aimed at folks in Scouting (here's one) might provide some good ideas. Watch out for antigay and anti-atheist stuff, of course. And if I were you, I'd steer clear of the loyalty oaths and circle jerks.

I think ghost stories must occur.

The Blair Witch Project, outside projected on a sheet.

(at least they will always remember that night)

Make a giant sheet tent in the biggest room of the house and then project the movie or slides or whatever (great idea) on it as a surprise. This also makes surprise animal attacks easier, too, as well as letting them use pillows and sleeping bags indoors.
Fire and ghost stories seem mandatory.
i was also going to suggest the ritual dismemberment of a pinata but it seem like promoting unnecessary violence. Making it the attacking monster that is filled with camp themed treats seems more justified.

Now i wanna go camping.
With a fire.
Or maybe just committing arson.

Well, two things I loved about Scout camping were these ways of cooking stuff in aluminum foil, both delicious:

Hobo Dinners: Everyone makes their own, so it's a fun little activity as well as a meal. Put some ground beef or Italian sausage on a large square of foil. Add chunks of veggies cut relatively small: potato wedges, onion slices, 3" ppieces of corn on the cob, baby carrots, peppers. Sprinkle with seasoned salt and stick right into the fire ashes to bake. Retrieve with tongs, open foil, add BBQ sauce or Tobasco or whatever you want, and yum.

(This also works in grownup version with things like sweet potato, soy, and scallions).

Dessert on the same principle: Give everyone a banana sliced lengthwise down the center. Lay it on foil. Then present them with several bowls of junk food which they can sprinkle onto the banana: maraschino cherries, M & Ms, crumbled cookies, butterscotch pudding, coconut, pecans, etc. They glom junk food onto the banana, wrap the packet up, and again roast it in the coals until everything's melty, and eat with a spoon.

By the way, your theme is GREAT.

I wish I could favorite this post. :)

Sleeping bag sack race!

How about a simple orienteering treasure hunt (6 paces N, 3 paces SW, etc.)? I always liked using a compass.

Is it a sleepover? If so, shadow animals are a MUST.

And how could I forget: SNIPE HUNT!

Maybe I overstate my case, but is this non-opening of presents something they do elsewhere, too

It's the norm here. I like it. It saves time and eases the present-buying competitiveness among the alpha moms.

Great ideas--thanks friends! I am so doing the bear attack. I'll set it up, pretending to hear a bear and mentioning bears beforehand. I love scaring the Bejeesus out of small children.

Update: It poured down rain all day yesterday, so that eliminated my nature walk bingo cards and some other planned activities. We made smores in the microwave and played twister inside the big cabin tent in our living room and played a game identifying nature sounds that I pretty much made up on the spot. It would have went pretty well except...

...there were two just horrible little boys who came. My fault, we did not get out the invitations early enough so had to do it at school. The rule at Wonderboy's school is that if you hand out invitations in class everyone has to be invited. I thought that would be OK, that just Wonderboy's friends would come. And they did but also two boys who never get invited anywhere any other way. Very snotty and foul-mouthed and aggressive and pushing the other kids around. This would have been manageable except that Wonderboy is also a very strong-willed and bossy child. We ended up with constant bickering that often led to tears at the end of the party. Yikes!

Thanks again for all the great suggestions.