After you return to your own time, you go back to 600 A.D. in your quest to save the future.
New Game +
Just by going to 600 A.D., a new ending opens up. Go back to the End of Time. Use the bucket, or if you're playing a New Game +, go to Leene Square and use the telepod to go directly to Lavos. ending "Legendary Hero"
Cross Zenan Bridge
Go to Zenan Bridge and talk to the guy in gold armor. Then go to Guardia castle and talk to the chef. Go upstairs and head for the door.
Before this next part, make sure Robo and Lucca know Fire Punch.
Go back to Zenan Bridge and talk to gold guy again. Give him the Jerky. When the attack begins, talk to him and say you'll help him, to get a Gold Helm.
Switch Lucca and Marle into your party. Go left. First beat the Deceased with magic (Lucca and Marle will kill them in one shot, but Crono won't, so leave him to move last), then use a physical attack on Ozzie. Go left to fight more Deceased, then hit Ozzie with a physical attack again.
After this second batch of Deceased, switch your party to include Robo and Lucca.
First kill the top half with physical attacks, then use a Mid Ether on whoever just lost their MP (unless it was Crono), then use Fire Punch on the bottom half, Crono healing, till it's gone. Zombor has a powerful drain attack which hurts for about 200 HP. Hard not to die from that one, so leave Crono to revive as necessary.
Go south to the town of Dorino. Go to a residence there where you'll see a sparkle. Check it and talk to the old man to trade your Naga-ette bromide for a Magic Tab.
You'll pass Fiona's Villa, stop in for a couple chests (both Mid Ethers).
Enter the woods north of Porre. Get the two chests there (lower: Mid Tonic; upper: Shelter).
If a Nu (round blue creature) appears, get into a fight with it onscreen, and you'll fight it too. It gives lots of experience. It's only attack is to take you down to 1 HP, it can't kill you. The other monsters can, though, so try to get rid of the other monsters first, and if any are still there, heal up if you're at 1 HP.
Then go up to the top, and climb down the bush in the middle.
Walk a little ways into the room, and Frog will appear. Talk to him if you want. Then open the chest (MagicScarf) and leave.
In Porre, go to the Market, there's a Power Tab in the corner.