You can still do Battle Frontier, I believe. Earn BP there to purchase useful battle items, TMs, et cetera.
You said you have all the legendaries? Make sure to get Mewtwo in the Cerulean cave and (if you have SoulSilver) Groudon in the Embedded Tower. You’ll need to have gotten a Kanto starter from Oak for this.
And although it isn’t legendary, you can obtain a Lapras every Friday in Union Cave.
You can also go here for guides, if you’re interested.