I've heard a rumor that you can train a Bayleef, Chicorita, and Cyndaquil..

Sorry, as this is a subpar answer for me, but I can't find my notes and cannot remember if you get one of the Jhoto starters when you fill your Kanto Pokedex, or if that's only in Ruby, Sapphire, and Emerald, and my LG is out on loan, but if you can, the only way would be to fill your Kanto Pokedex, then go see Professor Oak and he'd be the one to give it to you, unless it's the guy Bill who gives it to you. I do know there's a legendary dog running around Kanto waiting for you to find him!-he could be anywhere except the islands and water I doubt, too, but if you put a level 39 or so Pokemon in lead, use a bunch of max repels and you might find him easier-he'll be at level 40. Which one depends on what Starter you used-Squirtle-Raikou, Charmander-Suicune, ann Bulbasaur-Entei. But as far as the Jhoto starters, I cannot remember, but if you can you only get a choice of one-I know this works in R/S/E when you fill the Hoenn Pokedex. Besides going around to find other Jhoto Pokemon on the islands, showing Pokemon to that lady, & getting stickers from the sticker kid there's not much else to do except fill the Pokedex-you should be able to do a lot of breeding! And the Trainer Tower. Have you rebattled the Elite 4? Maybe you should try trading with a friend or if you have the equipment, get yourself Emerald or something and you'll be able to Trade after you beat that E4, but with R/S you can trade as long as your machine is completely fixed-if you cannot rebattle the E4, then you still have some things to do. You still need to get your Trainer Card up to Gold and with all the stars by filling Pokedexes and trainer tower wins probably. When I find my notes, I will try to give you a better answer as soon as I can though! There's always more to do, but you may not be into filling your Pokedex, but there's more to do when you can link with others-like Berry Crush with one other player and Dodrio Berry Picking if you have 2 friends with Wireless adapters. Linking up with other games can be cool though, even the GameCube ones, as you can see your Pokemon in a better view and they look more real, too. And how many shinys do you have? If none, then there's something to look for anytime, anywhere there are wild Pokemon or eggs to hatch! I think you should be able to find at least one in every game, but you may miss them as not all are obvious, but that's how it's been for me so far-I've found at least one in every game except my one second copy of Emerald I haven't really played too much.
Then, there's always cheat devices to bring your games to an new level, but be careful and use only as few codes as possible, but it can be fun to have all the stuff and Pokemon, too! The newest AR codes for FR are excellent for encountering all 386 in that game itself-preloaded on AR Max Duo! I keep trying to give you ideas, as I hate to see someone bored with a Pokemon game! FR/LG were OK, but as far as stuff to do, Emerald is the best as it's seemingly never ending, plus there's always contests and battle tents, the battle frontier, plus all the islands you can get to if you cheat to get Deoxys, Mew, Lugia, Ho-Oh, and both Lats. Kyogre, Groudon, Rayquazza, the 3 Regi, and at least one Lat you can get all without cheating, too! And the extra Safari Zone areas, too.
Hope I gave you a few ideas better than just catching Unowns-but there are 28 of them to find, too, though it won't do anything special as far as I know. I wish I had more ideas, but it's pretty late, and I'm without game or notes currently, but I will try to look into it. You can always go search through the walkthroughs to see if you missed anything, check hints and cheats to see if there's something you may have not tried yet. I've not been able to really finish a Pokemon game, but I go beyond the Main Story lines and go for it all, so that's me, but I hope you find more to do, too. Good luck!
I was looking more in my notes about the Jhoto starters, but I can't seem to find anything on that, so it may just be R/S/E that you can do that with, but if you have a full Kanto Dex, it's worth a visit to Oak and Bill-I think Bill will at least give you a certificate, or something like it. Sorry!

Well I have never heard of being given one of the Jhoto starters. And only yesterday I finished both my Kanto and National dex. I reccomend the best way to get the Jhoto starters is by snagging them in Pokemon Colliseum and after they're purified Trade them onto your game so you may train them. It could be possible to also get them from Ruby and Sapphire, I have never got a perfect dex in that game, but it is true for Emerald i'm pretty sure.
After you beat the Elite 4 and repair the trading machine it is true there isn't much left to do but fill up your dex. Of course there are the islands with Ho-Oh, Lugia, and Deoxys you could go to but you need to use a sort of cheat for that. I have never gotten the islands before so don't worry to much. But once you have nothing to do you can always begin to level your Pokemon to level 100. And then find someone to battle.