Well, if you wanna catch the legendary Pokemon, here is a good li'l list of them. Knock yourself out.
As for other events, you can:
- Catch all the Pokemon in the Johto Pokedex
- You could beat Red (if you haven't already)
- You can set the goal of beating all of the frontier brains of the Battle Frontier
- You can complete all goals of the Pokeathlon (get the highest score in every Pokeathlon event)
I guess that's kind of all. There's probably all, but those are probably the most notable.
Hope I helped, and good luck. :)