These will blow the mind of a child on Easter morning and is a fun afternoon project. I made these for a video I shot. We needed large eggs that I could put a hand inside and break out of, and these worked nicely. But I also found that they were fun to make, great decorations and kids love them. Thanks to Tom Cline for showing me his Giant Plaster Egg 12 years ago.
WARNING, this is easy to do, but tough to master. It will take several attempts. Be patient and do this in an area you don't mind getting dirty. The eggs tend to explode when you are making them, (nothing dangerous, but messy) and you will get plaster everywhere.

Step 1: Materials

Plaster of Paris-one box.
Bag of 9" balloons-round (but really Egg Shaped) clear is best, or white or yellow, easy to see through.
Empty bottles of water-1/2 liter. Easy to crush.
Mixing cup, more than 1/2 liter.
Mixing stick
Water
Pillow
Exacto Knife or Razor Blade
Sandpaper
Spray paint
Towels, drop cloth, (did I mention this is messy?)

Step 2: Mix Your Plaster

The mix should be a bit runny, like a thin milkshake, it should pour easily, like syrup.
This is important; too thin and the walls of the egg will be too thin, too thick and it won't spread around enough. Too much water and it will take a very long time to harden. All I can say is try and try again and know it won't work the first time.

In this picture I use a piece of 1/2" PVC pipe. Not a good choice, next time I used a screwdriver to mix. Try to remove most of the lumps. I bet a paint mixer on a drill would work well.

Step 3: Pour the Mix Into the Plastic Water Bottle.

The reason for the water-bottle is to force as much mix as possible into the balloon.
A funnel will not work to get the mix into the balloon, but it would work to get the mix into the plastic bottle.
The bottle should be full or almost full of mix.
Very messy work.
Rinse bottle off.

Quickly go to next step as the plaster is hardening as we speak.

Step 4: Squeeze Bottle of Mix Into Balloon

Place the balloon lip over the opening of the bottle.

Hold the balloon on the bottom. With the other hand squeeze the bottle like a tube of toothpaste and get out as much as possible into the balloon. Careful, the balloon can tear at the neck and you would have to start over.

Carefully remove the balloon from the bottle.

Throw out the bottle, it can not be reused.

Quickly move on to the next step, plaster getting harder and harder.

Step 5: Blow Up Balloon

You may want to quickly rinse the balloon off, plaster does not taste good. Blow up the balloon to the desired size. You might try blowing the balloon up and letting the air out before you start step one, just to make sure it is stretchy and no holes.

Careful, the balloon could pop and you'll have plaster everywhere. And you don't want to do it in the garage, where your wife put the baby car-seat, and that nice picture of our wedding and some throw-pillows she was meaning to bring inside any day now.. Because if you get plaster on that stuff...well, I wouldn't want to be there.

After the balloon is blown up, tie it off and quickly go to the next step. Plaster, drying, times a wasting.

Step 6: Rotate the Balloon

Rotate the balloon quickly at first to cover the inside. Here is where having a translucent balloon will help see where the plaster is inside. Get it completely covered and keep moving it. Look for streaks and weak spots and move the plaster to that area. Once completely covered, keep moving the balloon slower and slower to keep up with the ever slowing plaster.
The more even the coat is inside the better. You can carefully tap the balloon lightly with your finger to keep the plaster moving. Feel where the plaster is the most and move that part to the top to slowly settle over each side.

As soon as the balloon feels a little bit hard. STOP. Set it on a pillow. You can rotate it a few more times maybe a quarter turn every minute.

Step 7: Let Set, Dry, Harden

Let it set a couple of hours. This takes a bit longer to completely dry than other plaster projects since it is completely enclosed in the balloon. The balloon can still burst at this point.

Step 8: Remove Balloon and Decorate

With an Exacto knife or razor blade, lightly run the blade on the balloon. Just enough to cut the balloon but not the egg. It will break away with a rip. After a couple of cuts you can remove the rest with fingers.

Step 9: Decorate

Sand down the tip where the nipple of the balloon made a non-egg like mark.
Give it some more time to dry without the balloon.
You are ready to decorate.
Let your imagination run wild here. Spray paint works. You might try a primer coat to make it smoother, less porous. I want to try painting it, covering part with masking tape and painting a gain to get stripes. In the pictures I just painted it Silver and Pink.
I really like the metallic colors. Gold has worked nicely. And there is the nice imagery of the Golden Egg.
I'd love some ideas on decorating, this is not my strong suit.
Check out the painter pyramid instructable I did www.instructables.com/id/Painters-Pyramid-cardboard-version/.