This April Fools joke is probably my personal favorite.
This joke is more for guys.

Step 1: Supplies

1. some clear plastic wrap
2. 1 roll duct tape
3. 1 pair scissors
4. A toilet

Step 2: Preparation

A. First, lift both of the toilet seats up as in picture 1.

B. Next, you will need to cut five duct tape strips about 2" as in picture 2.

C. Then, place a sheet of plastic wrap lengthwise on the toilet as in picture 3.

D. Now, using your duct tape strips, tape down the plastic wrap as in picture 3.

E. Finally, close the first lid and admire your work.
The finished product should look like picture 4.

Step 3: The Surprise

This will provide a big surprise for the next male who comes along to urinate.

Step 4: Even More Toilet Pranks

Do you have a great toilet prank idea?
If so, then write your idea in a comment on this Instructable.
And probably you will find your comment on this list.

f you take the lid off the tank you can take the hose and point it out the back or front of the lid. Be sure the hole is clear of obstructions and that it isn't obviously sticking out. When someone flushes the toilet, water will squirt out and not stop until the hose is returned to the tank and the tank fills up. Obviously, this is best done at a friends house or (the best) at a bar/bowling alley.

If done in a public place, some people like to give a "courtesy" flush. This will result in them getting showered with water (back of tank setup) or sprayed in the back (front of tank setup).

I've done this one for years and it works great. From Harleydreamer

Put folded ketchup packets under first seat when sum1 sits down big explosion
From cheezydog10

Heres another trick get party confetti things and take out the pull fire cracker and attach the string to the toilet seat and tape the fire cracker to the toilet put the toilet seat down when a guy goes to pull up the seat to pee BANG! From struckbyanarrow

Get gelatin and boiling water,
Mix it together real good,
Put in toilet and stir with brush!

You have to scoop out your poo!!!
From Duck-Lemon

Baking soda in the bowl, and vinegar in the tank.
When they flush, it produces CO2 and overflows. From bruno13069

You could always make it so that it is impossible to flush (maybe disconnect the flush handle from the inside). Then you could make fun of them for "breaking" your toilet.

You could also add some red food coloring to the water tank to give the flusher a surprise when he/she finishes. You could also make it brown and act disgusted when they leave (make sure to create a huge scene). These are just some ideas I thought of sitting here looking through the April Fools pranks.
From Dr. Paj

You should do this in a public bathroom, and wait in the next stall for someone to go. From Domindude3

In college I used to put a couple throw pops (like they sell for kids on 4th of July here-you just throw them on a hard surface for a bang) under the toilet seat risers, so when someone sat down you would hear an explosive sound echo from the dorm bathroom! Guaranteed every time that if they didn't need to go before-they definitely had to go after!
From inquisitive

Take all the light bulbs out of the bathroom and leave the seat up, that ones for the ladies, although i wouldn't do that to my girlfriend unless she did something really bad to me.
From _Ko0LaiD_

Or try putting Potassium Iodide in the bowl, then a bunch of 30% Hydrogen Peroxide and a cup or so of liquid dish detergent in the tank. The Potassium Iodide catalyzes the separation of the Hydrogen Peroxide (H2O2) into water, oxygen, and heat... and lots of it. The detergent captures the oxygen and steam in bubbles. This means the toilet becomes a volcano of hot foam.

WARNING: the foam is very hot, and goes everywhere. It's not like they cant run from it, but it could scald, and will take quite a while to clean up.
From Kooshi_Govno