RE: Wha to do when you beat the E4 in Pokemon Red
After catching the legendaries and beating the elite 4, you can capture all the pokemon possible in red, and then battle to evolve all the ones that evolve by lvl and use stones on the others. If you have blue, and can trade between paks, get all the blue exclusives and trade evolutions. Then, if you have a N64, you could get a transfer pak and pokemon stadium. You can actually transfer Pokemon onto the n64 game and then onto the GB game, thus eliminating the need to trade between game boys, lol.
Pokemon stadium 2 is technically the better game, since it has more battles, and much better mini games. However, if you're like me and enjoy the 1st gen gameplay mechanics (the single special stat and lack of dark/steel) then Stadium 1 is a good choice for the complete RBY experience. Personally, I just love being able to dominate with Hypno in RBY XD Of course, if you REALLY wanna maximize your RBY game, you could play through stadium 1 AND 2, both rounds each game. (That's alot of battles...)
So yeah, that's the extent of everything possible with RBY! Just to note, the Stadium games are MUCH harder then the GB games. Especially Stadium 2, I have a hard time with that game even using hacked pokemon!