What about expanding things that you would normal have to get a microscope to look at (such as atoms) and seeing if their physical properties are any different than the models in science classes (if this makes any sense).
One thing that I’ve been planning on doing the next time I have a vivid LD is go on an epic quest to find a special music box. When you open it, it plays beautiful (hopefully original) music. I haven’t exactly thought up the specifics, but it should be an interesting expiriment.
Something I strive to do and think would be interesting would be to map out your mind as a physical place, be it a building, a city, or even a planet. Explore the Caves of the Subconscious or even take a literal trip down Memory Lane.
I just thought of a good idea. For all you music players - why don’t you play with your instruments and try to create music that becomes alive?
I’d like to see the results if someone tried those and normally doesnt play an instrument.
Find out what your Daemon looks like and it’s name.
For those who haven’t read Pullman’s books.
A Daemon is part of your soul, represented by a talking animal,
in most cases of the opposite sex.
While you are a child they daemon can change shape all the time but when you grow up it will lost the abilty to change and became one animal representing you for the rest of your life.
The Daemon is a really good one, I’d do that. In fact, I’ll probably do it even if it’s not a quest.
Elemental quest - Play with the elements! Fire, Water, Air, Earth and Aether (AKA Spirit). See what you like best, etc. We can consider also different philosophies on the elements, such as the Chinese one, with Metal and Wood 
-Rearrange the galaxy! Don’t you hate Neptune being so far away? Just push it closer to the sun and enjoy the view from earth!
That’s what I did last night. I accidently burnt Uranus to a crisp, Venus and Neptune crashed into eachother and the gasses destroyed the earth…
-Alchemy, Fullmetal Alchemist style! Clap your hands together and touch the molecules you want to rearrange!
I built a house this way. And I made a tree grow, but the tree became too big and blocked out the sun. All the DCs died. Whoops…
-To the center of the earth! Drop through the floor, all the way to the centre of the earth, keep falling and come up again on the other side.
Fun, but molten rocks hurt!
-Play sports with DC’s!
Just make sure they don’t fall off their brooms…
for those people who like board games and adventure:
play Jumanji.(remember the movie Jumanji)
Make someone believe that he is just a character in your dream! I tried it, but they just gave me weird looks… Maybe you will have more luck.
How about meeting up with a deceased friend, relative or pet. I often dream of my grandparents, and my deceased cat who passed away a year and a half ago 
. Unfortunately, I rarely become lucid when I dream of them. I would love to hold my cat again 
He lived to the ripe old age of 18!
10 random suggestions off the top of my head:
Animorph
Take lsd
Go back in time\forward in time. (eg: See what your life’s like in 10 years or something)
Sit on a cloud.
Make everything upside down
Take your skin off and walk round the town centre.
Eat anything and everything. yes everything. eg: shoe, mobile phone, even vegatbales.
Hear through your nose, speak through your ears, see through your mouth and taste with your eyes.
Jump across roofs.
Go to the bottom of the ocean.
Cook something.
Well I think this would be really, really, unbelievably hard, so not that much help maybe - turn your body into a swarm of bees.
Basically try to control lots and lots of bodies with one mind. I also think it would be interesting to do it with bees because they have useless individual bodies and that will make it interesting to see how we will cope when we to do normal things.
My suggestion:
Poke a Tiger in the eye!
Or you could hug one 
Or go on Safari and do the above too!
Fleep
Try and see colors you can’t imagine.
Ok that is a paradox, I mean try and see colors you can’t imagine in waking life.
Try that not only with colors but with sounds, feelings, taste, and things.
I think I’ve said this before :
Try to enter a parallel universe
Also- try to go back in time and watch yourself grow up.
-Live the fairytale. We’ve all heard and read them when we where younger, why not relive them and maybe change them a bit? Buy a new pair of boots and save the fair maiden!
-Its a small world. Shrink yourself to the size of an insect and explore their world. Talk poetry with butterflies, hop on the back of a grasshopper or visit a snail’s cosy home.
-The Lucid Travel Agency. Give a DC a tour of your dreamworld, show them the things that make your dreams special. And while your at it, pick an interesting way to travel. Brooms, airships, dragons, the catbus, whatever suits your tour best!
-To the end of the world and beyond! There a many theories about the end of world, why not see for yourself? Go to the end of the world and find out what happens. Or vitit the day after the end of the world, if you don’t believe the world will ever end.
Make music
Create the sky
Go to LD4all
Cook some food
Climb into the television
Be an animal
Get on one of those drop rides, only on this one you can fall indefinitely(however long you want) before slowing down and stopping to get off. Remember to feel everything you’d feel on a real drop ride. Try to hold conversations with other riders while falling.