After rescuing Crono, you can do optional sidequests or go directly to the final boss battle.
Chance for an Ending
If you defeat Lavos now, you will see the ending "Beyond Time".
Prepare for Sidequests
You're automatically sent to the End of Time with Crono. Now that he's back, he's not a required member of your party, meaning you still have access to those other triple techs that opened up.
Before you go for the side-quests, you'll want to go to the Laruba Ruins in 65 million B.C., and there's a Nu all the way at the top. The Nu will give you the SilverRock, opening up the Spin Strike triple tech. The Nu will also change the name of whichever character is in the lead. (You can cancel if you don't want to change the name).
Also in 65 mil B.C., the trading hut is now offering Ruby Armors, which cut fire damage by a whopping 80%. To get one (and I recommend only getting one, since you should have the Red Mail and Red Vest already), you need 10 each of all the four trading items. The quick way to get these items is to fight the Nu in the Hunting Range as outlined in "Footsteps! Follow!". But by this point in the game I had a ton of the trading items.
Back at the End of Time, talk to the old man if you want, he lets you know about various things around the world that can be done (side-quests).
These are the sidequests that can be done:
- The Sun Stone - Complete this quest to get powerful armor
- Ozzie's Fort - Complete this quest to get powerful weapons and armor
- Cyrus's Ghost - Complete this quest to greatly increase the Masamune's power
- The Origin of Machines - Complete this quest to permanently increase Robo's stats and get his most powerful weapons
- The Sunken Desert - Complete this quest to get powerful items
- The Rainbow Shell - Complete this quest to get powerful items