*Do you know other super informal words *mean* the same? Interesting that you pick American English. British English is a lot more standardised I would say...then again you are looking for colloquial terms. Maybe 'fab' which is short for 'fabulous' or 'wicked' which doesn't mean 'evil' in this instance. Good luck!

I could give you lots of synonyms from my generation, which you don't want to hear, because they're outdated. But I have six sons, your age or younger, and I could suggest two equivalent terms to "sweet!". Those would be "sick!" and "dope!". You could also try looking at "urban dictionary" for other suggestions, though I would caution you that a lot of the words found there might be too vulgar and even "too slangy" to consider. using. "Sick" is probably the best suggestion I have. Have fun! :)

There was a slang word used in Australia and NZ a while ago, which was "grouse". It basically meant great. However, like a lot of fashionable slang, it soon went out of date and became an embarrassing (or "daggy") thing to say. I've also seen "filth" come back into fashion, to mean positive approval.

awesome!

Some British teenagers use the word 'bare' to mean 'really' or 'very' i.e. That's bare good = that's really good. I'm bare tired = I'm very tired and so on. God knows where it originated from, it makes me cringe nowadays but there was a time when I used it myself lol. 'Sick' did the rounds here as well as a word that meant something really good, I remember hearing it a lot when I was a teenager. A cruder alternative might be describing something as 'the dog's bollocks' which means something is really fantastic. You have to be careful with that one as 'bollocks' is also a slang term for testicles and if someone is 'talking bollocks'' it means they are talking nonsense.