What's a word for "a thoroughly enjoyable experience"? Something like the opposite of a "trial"?
The condition or sensation induced by the experience or anticipation of what is felt to be good or desirable; a feeling of happy satisfaction or enjoyment; delight, gratification. Opposed to pain.
Something highly enjoyable; a great pleasure, delight, or gratification.
Anything in which one takes delight, or which affords delight; an object of delight; a source of great pleasure or joy.
(All from the OED.)
"Slang" terms that I've come across for "pleasurable" are "lark," and "lulu."'
That was a lark. or That was a lulu.
If you mean something that gives pleasure: Delight
a high degree of gratification : joy; also : extreme satisfaction
something that gives great pleasure
If you're talking about the state of mind: Bliss
There are also joy (which can be used in both contexts), pleasure, and felicity.
A good opposite of trial would be lark or breeze, if you are interested in the testing or "putting to the proof" aspect of the word trial.
How was the job interview? It was a breeze.
You could have a ball (sense 2).
How about 'I'm having a blast!'?
When you're having a really hard time it can feel like a trial. When you're having a really good time it can feel like a festival.