Level Them Up

In early versions of D&D, characters would earn experience points for gold pieces gained. Although this wasn’t the most popular rule, it did serve a useful function: it brought characters’ innate powers “along for the ride” with sudden material success. In this way, it helped keep the game balanced.

Right now, you have a 6th level party equipped like a much higher level party. If you throw monsters/challenges at them suitable for the typical 6th level party, that can be a cake walk. If you throw much stronger monsters at them, they might easily be killed.

If you give them a couple more levels (over the next few play sessions) their innate abilities (notably, hit points) will come more in line with their equipment, and on-level monsters (or monsters of slightly higher CR) will pose an appropriate challenge.

Get familiar with their new equipment

You’ve discovered it can be challenging to run a game for well-equipped charactrscharacters, but this same problem can happen whenever characters gain magical powers, such as spells. (Fireball can wipe out an interesting encounter in a single round. Spells that allow characters to read thoughts can short-circuit mysteries.)

To keep things interesting, you’ll neeedneed to get familiar with the ways your characters can subvert the obstacles that used to be challenges, and switch things up appropriately. (They might be able to just walk up a castle wall, but finding the portal to the Feywild can still be a challenge.)

Read upa Published Adventure or Two

The ways to make trickier encounters suitable for more power characters are not necessarily intuitive. But published D&D adventures provide lots of example to borrow from, or at least provide inspiration. Out of the Abyss and Tomb of Annihilation are two challenging adventures that come to mind.