Things i can’t miss in FFIX, please help

I didn’t find it anywhere and i don’t want to read everything from the walkthrough, i think the game loses alot of fun, so is there a list of things that can’t be undone, things i can’t go back and do again, things i can’t miss?!?
I only found the "Gimme cat", i think there are some other things too, and i made a list of weapons i need to buy to combine later.
Help me please, thanks!

Don’t miss Ozma, Ultima Weapon II or Rebel Lance.

Don’t think that’s really what the original poster meant, Techchild.

Gubert, there’s a boss called Tantarian in Alexandria Castle that you’re only given two opportunities to fight. Make sure to get it either the first or second time (second time would be a lot easier), or else it’s gone forever. Also, after Disc 3, certain sections become inaccessible or much more difficult to get to. So it’s advised to complete or mostly complete certain sidequests (there’s at least one that can’t be fully completed until Disc 4) before moving on to Disc 4.

Ok thanks Agent0042!

Don’t miss Ozma, Ultima Weapon II or Rebel Lance.

And don’t attempt Ozma without power-leveling! Or you will be butchered almost instantly!

And don’t attempt Ozma without power-leveling! Or you will be butchered almost instantly!

I find it quite easy @ level 55 with the right abilities.