Hi, I am new on this, and I have had Pokemon Ruby for about a year now..

After you beat the Elite Four, there are still a few things left to do. 1) First of all, use the ferry ticket you got from your dad to ride the ferry from slateport or lilycove to battle tower. At battle tower, you can defeat lots of trainers that all have their Pokemon at level 50 or level 100. You can get prizes and improve your trainer card by doing so.
2) Also, try to complete your Pokedex by getting Sapphire (if you don't already have it) and a link cable. Try finding someone who already has both- it's a lot easier! I learned that from personal experience, as I already have all 200 Pokemon.

Now... The Legends...
I see that you already have Rayquazza, and you must have Groudon as it's a key element to the game. Not to fear, there are still more legends for you to catch. First of all, theres Latios; a rare psychic bird that appears randomly after you beat the Elite Four. If you haven't already encountered him, then here's what I recommend that you do.
First, buy a bunch of Max repels and ultra balls. Then, catch either a Golbat on victory road or a Trapinch in the desert. if you have the Golbat, then train it into a Crobat by making it happy and then raising it a level. I attached the soothe bell to mine, which helped a lot. Make sure it knows mean look and is at least level 60. However, if you have a Trapinch, train it to a high level- at least level 40, but make sure it doesn't evolve or it will lose it's special ability. Use either of these Pokemon when you encounter Latios. You can track Latios down on the Pokedex, with the area function if you've already encountered him. If you haven't, put a Pokemon that is at or under level 40 in front of your party and use a max repel. Then, search all the grassy areas continually until you either find him or run out repels. If you do run out, buy some more and continue the search. Eventually, you two will meet and then you can track him down using the Pokedex! This is still hard, but, hey! I've caught both Latias and Latios using this same method!

Now, there are still three legendary Pokemon left. The Regis. First, you need to catch/evolve a wailord and a relicanth. Then, you need to have pokemon in your party that know surf, dive, and dig. After you complete those requirements, go to Pacifidlog town and surf into the rapids left of the town, keeping to the south edge. About 3/4 down the routes, you will see a small diving spot sorrounded by 4 small rapid squares. Maneuver around these, and dive down. Cruise underwater until you come to a small patch of brail on a wall. Touch the brail, and dive up in that exact smae spot. If you surface anywhere else, you will just end up where you started. If you do it correctly, however, you will enter a large chamber with many brail stones inside. Ignore these; they do not matter. Instead, go directly to the end of the chamber, stand in the middle of the brail writing on the wall and use dig. The wall should bear a hole afterwards. Go through it. There you will find more brail. Make sure that Relicanth is the first in your party and Wailord is the last, and touch the middle brail. Something like and earthquake will occur, followed by three loud thumps. This means the tombs of Regirock, Registeel, and Regice are now open!

One is in the desert, another on the route between fortree and lilycove, and yet another on the water route to dewford from Petalburg. In order to open the one in the desert, go up to the brail. then, go 2 steps right, 2 steps down and use strength. To open the one on the water route, go up to the brail, press A, then leave the gameboy ALONE for a couple minutes. If you do it correctly, after about 3 minutes the chamber will open. In order to open the last one, go to the middle of the chamber and use Fly.
I hope this is useful! Gotta go now now, my fingers are achin'....

Sadly, you cannot reach Kanto/Johto in Ruby/Sapphire, but there are many things to do after defeating the Elite Four-in fact, too many to type down. Some examples are searching for shinys, breeding stronger pokemon, and determining what TM moves pokemon can learn naturally (for example, kecleon learns thief as a startout move and nincada learns dig at level 45).
There are 6 legendary pokemon in Ruby Version-Groudon (Cave Of Origin), Latios (any route), Rayquaza (Sky Pillar), Regice (Island Cave), Regirock (Desert Ruins), and Registeel (Ancient Tomb).

Well, you can finish the pokedex, get 5 paintings in the art museum, look for all sixty-nine trainers on the trainers eye, beet 100 trainers at battle tower, and get as many fans as you can.

The legondary pokemon are, groudon, witch you probably already missed unles you caught it. You know that big red thing in the cave in sootopolis? If you k-o'd it, you cant get it back.

There is also regirock regice, and registeel. For these you have to go west of pacifilog and go into the right currnts to get to a diving spot. Then you have to go up where there is some brail on the wall. When you get to the cave, use dig at the verry back where there is more brail. Then there is another room, with more brail. For this you have to a relecanth at the first slot in you party and wailord at the last. Press the a butten at the back of the roomand the other three chambers will be opened. Then you have to get to the three chambers with the three regis.
One is on the rout east of fortree, another is at the bopttom of the dessert, and the last on is on the rout north of dewford, and southwest of pettleburg. to get to the pokemon inside them, you have to do certain things in each chamber.

For one, you have to go to the middle of the chamber and use fly. For another, all oyu have to do is waight for a wile. And for the last one you have to go to the back of the room where the brail is, and go down, down, right, right and use strenth. I forgot witch ones you did in each chamber, but all you have to do is experement.

I hope this wasn't to confusing!!