Mars will show some surface detail as black stains and its Polar Ice Caps look like white stains. The “red” color of the surface is clear.
Phases of Venus are easy to observe.
Neptune and Uranus are observable as blueish dots, including Neptune’s largest moon Triton.
The Lunar surface will show a ridiculous amount of craters and mountain ranges with on-point color accuracy.
Overall the planetary performance of XT6 is second-to-none. It is comparable to the more expensive 8″ models due to its f/8 focal ratio. If planetary detail is your priority, XT6 is a no-brainer. BUT, you should also check out SkyWatcher 6″, since it comes with a 2″ focuser, it is more future-proof.