It lights up when you open it, or maybe that’s just dramatics. Can anyone gather what’s inside given the conversations in Brad’s apartment, subsequent events concerning Marcelus Wallas and perhaps Mia, and Jules’ and Vincent’s reactions upon looking?
Forgetting about fan theories (it was the Maltese Falcon! It was the Holy Grail! It was Marcelus Wallace’s soul!), and keeping in mind that the actual narrative point was to have some sort of meta commentary about the use of MacGuffins in movies, logic says that a bunch of underworld types would keep either money or drugs in the suitcase.
Because everybody acts quite impressed when seeing it, it was probably the second, some sort of rare extra pure heroin, for instance.
I had to have been non-monetary to be lying inside a suitcase for sometime. If it’s something hot like drugs or cash, Brad and company wouldn’t be lying around as one group waiting for Wallace’s hit men to come pay them a visit. And Vincent’s reaction was not typical of a drug user seeing a big stash (he would have smiled.)
Another, smaller suitcase.
Reading about it, the most enjoyable theory I’ve heard is that it was the diamonds from the botched heist in Reservoir Dogs. There’s the rumour that the original script mentioned diamonds until Tarantino chose to leave it as a mystery,
Diamonds (or other gems) make sense: They’re worth a lot, but only to the right buyer, they’re impressive to look at, and it’s not too much of a stretch to depict them as shiny.
But I like the modern idea that Jules was a cover identity for Nick Fury, and the suitcase contained the Tesseract.
When it’s opened (twice in the film), there is a golden glow coming out of the thing.
The contents are described as “beautiful” by Honey Bunny, which destroys my theory that it’s a working fusion reactor. 
I also think (not sure!) that she uses a singular pronoun - “It’s beautiful.”
Also, it’s pretty light, so that doesn’t help the “it’s full of gold” theory.
My guess: a stolen piece of art.
My theory: The kids stole it from a museum/their college, then decided to keep it themselves after they got paid by Marcellus.
My odds of being right: 0%
Lite Brite
I’m pretty sure she would say “it’s beautiful” if she saw a suitcase full of jewels, referencing the content as a whole.
I like to think of it as that little tiny universe from Men in Black.
Possibly. I also (hazily) recall Tim Robbins character asking “Is that what I think it is?”
However, I’m at work and can’t check it out - sets a bad example for me to view Pulp Fiction clips in front of the employees.
As Sr Siete said, it was supposedly diamonds in the script, but Tarantino changed it because he had just used diamonds in Reservoir Dogs. So it was nothing - simply a McGuffin. His co-writer said it was a mistake to add the orange glow because it made people think it was actually something.
It was John Travolta’s marbles.
I’ve heard the “Elvis Presley’s shiny golden stage outfit” hypothesis before.
The word “beautiful” would support an argument against that hypothesis… but then again, maybe everybody has bad taste. 
It was…wait for it…Marcelus Wallas’ soul!
Boom. Mind. Blown.
#1995flashback
Well, not everybody; Honey Bunny.
And it’s practically canon that Honey Bunny has bad taste.
The new, yet to be released iPhone.
Tim Robbins? Andy Dufresne turned to a life of petty crime after busting out of Shawshank?
But actually it’s Ringo, not Honey Bunny who says “It’s beautiful” with awe in his voice. Honey Bunny is on the other side of the diner when it’s opened.
Too late to edit:
Here’s a less-edited video, jump about a minute in. Ringo says “Is that what I think it is?” Jules nods “Mm-hmm”, and Ringo says “It’s beautiful.”