How much planet detail will I see with a Meade ETX-125PE?
I've had an ETX-125 up to 200x on Saturn on a good night and was able to see, not only the Cassini division in the rings, but also the shadow of the rings on the planet. Also some very subtle surface detail. Jupiter will be no problem on cloud belts and you should be able to see the spots, but is currently too low for good observing. Will get better this summer, though, as it climbs higher in the sky. Mars is not going to do much for you, right now, due to its distance, but when closer, you will see surface detail and polar caps easily.
If you really want to pursue them, you can see Uranus and Neptune, but, no, not going to be much more than points of light. These two are really beyond the grasp of amateur telescopes. Stick to Saturn, Jupiter and Mars and you will be happy. The ETX-125 is a great planet scope and don't forget the moon.
First thing is always to add more eyepieces. An eyepiece kit, such as the Meade ETX kit.