Hidden Room in Radat:
To get to the secret room in Radat you have to be mistakenly teleported there by Viki from the headquarters. There are some great items there which you can get there.

Instant Level 99:
Go to L'Renouille when a character has reached level 39 or reached level 49 if they have a Fortune Rune. Kill off ALL the party members but him and defeat the group of six Highlanders and his level will rise to 99.

Unlock Extra Flowing and Mother Earth Runes:
Go to Gregminster speak with the Rune Master and purchase ALL the runes that appear in 'Rare Finds'. Select 'Quit' if there aren't any 'Rare Finds' and look at the cabinet behind the Rune Master and talk to him again. There is good chance that the 'Rare Finds' will open, it's worth trying this once every time you go to Gregminster.