We did this one for my daughter's fourth birthday on a hot day in the garden, and it was the easiest party I have ever thrown. Almost all the decorating is fabric.
The fishing pool did most of the entertaining. Children came back to it again and again between everything else, which is exactly what you want from a party station.
Everything blue counts as decoration. Blue plates, blue cups, a blue sheet. Nobody is inspecting the details.
Tip
Do the jellyfish and the jelly cups the night before, and leave only the fabric and the pool for the morning. If the weather turns, everything except the pool moves indoors without losing anything.