After you return to Leene's Square, you should go back to Guardia Castle.
Back in Leene's Square
You will be back in Leene's Square at the Telepods, and there will be a Gate that you can check on if you want to go back to the past.
If you're playing a New Game+, you can
check the sparkle in the right-hand telepod to go directly to
to the final boss.
If you win, you get the ending "The Successor of Guardia".
Go to Guardia Castle
You can rest for free in Crono's bed if you want to. You can optionally introduce Marle to Crono's mother. When you meet new characters later, you can introduce them to Crono's mother, too. Now take Marle back to Guardia castle.
There will be a cutscene. When asked, say that you started everything, and say that you weren't tempted by her fortune. Eventually, you will see the results of the inquiry.