This goes a step further from dixie cups and goldfish.
In this instructable I will tell you from personal experience how to suprise your victim(s) with a live salmon in a canoe.

There are a few basic things you will need to accomplish this prank:

1. Canoe
2. Fish
3. Bucket
4. Opportunity

You may also need

5. A screwdriver to remove doors or windows to get the canoe inside
6. Small pieces of wood to prop up the canoe to keep it upright
7. Tape-measure to ensure your vessel will fit
8. Friends to make it much easier and faster

Step 1: Determining the Perfect Location

Once you have chosen your prey scouting the location may be obvious but should not be under-analyzed. You must approach this hastily so as to not get caught yet still taking the time to thoroughly assess the plausibility.
Making sure the canoe will fit in the room is key depending on the amount of furniture and exact length of the boat. You must also check to see if there are any tight corners or hallways that might not be maneuverable. In this case, you will have to resort to a window checking to see if there is one big enough and will open far enough.

Step 2: The Boat

Get a canoe, make sure it doesn't leak otherwise they'll have a giant fish tank and a wet floor.

For this we used one of the aluminum crafts shown in the picture that worked quite excellently.

Depending on your canoe you may need to use pieces of wood on each side to keep it from tilting one way or the other.

Step 3: The Fish

Catch a fish.
The amount of water will depend on the size of the boat, we put about a dozen 5gallon buckets of water in our boat. It should be about one third of the way full or just enough for your fish. Using water from the the place you caught it will be easiest especially if its salt water.

A garden hose would also work for fresh water and would be less work.

Step 4: The Reaction

Now you just have to wait for the discovery.

Yes, they will be puzzled and they will take pictures.

Now I may be a gentleman in most cases but I found it best to let them figure out how to remove this seeing as only the best of pranks are executed without assistance in the aftermath.

....these victims had a live salmon in 60 gallons of saltwater straight from the ocean in their room all night before removing it the next day.