At some point, you will need to hunt for the CUBE in the Waterfall. You can come here and loot the place as soon as you have the AIRSHIP, and it's not a bad idea to do just that. The treasures here are fantastic, and it's a very brief dungeon.
To reach the Waterfall, land the AIRSHIP near the river that flows past Onrac, then CANOE up the River to the Waterfall to the north. The monsters found in the North Rivers are much more powerful than those found in the South. In particular, GATORS and FrGATORS have dangerous physical attacks. NAOCHOS can also deal a lot of damage, and combine their offense with solid defense. If you've waited until you've been to the Mirage Tower before coming here, you should have no problem dealing with these battles.
Waterfall
The Waterfall only has one floor with a single treasure room. It has a confusing layout, but you can reach your destination easily by following the southern wall as you head west. Most of the encounters in the Waterfall are surprisingly easy. NITEMARES appear often but lack any way to really hurt the party, and MudGOLS can be taken out while they're still buffing each other with FAST. WzMUMMIES are easily handled with HARM spells or Fire, and PERILISKS are best taken out with LIT2 or ICE2.
The one very dangerous encounter in the Waterfall is mercifully rare. You probably won't run into a Gas D, but you are in for a fight if you do. There is no A-spell for Poison, so the Gas D's POISON ability can devastate Warriors not equipped with a Ribbon or Aegis Shield.
The treasure room is guarded by a combination of WzMUMMIES, MUMMIES, COCTRICES, and PERILISKS. This battle is easily won with liberal use of attack spells. The spoils to be found here are immense. The big prize is a Ribbon, likely the second one you've found. You will also find Defense, a powerful and highly accurate sword that casts RUSE when used as an item. The accuracy bonus makes this a great weapon for NINJAS or RedWizzes. Finally, you'll find the Wizard Staff, a fun weapon that casts CONF for free. CONF isn't very effective, but it can be a lot of fun. The other chests contain nearly 25,000 G.
Don't forget to talk to the robot, who hands over the CUBE. With this, you can warp from the Mirage Tower to the Floating Castle. You'll have to head back down the River to your AIRSHIP, perhaps stopping in Onrac to rest first. Head east to the Yahnikurm Desert and prepare for a return to the Mirage Tower.