Due to its significant brightness (up to magnitude -2!), Mercury is easy to spot with the unaided eye, however a telescope will allow for documenting the apparent change in size over time.

While Saturn's brightness makes it easier to see, its relatively featureless atmosphere means that less details can be resolved than with other planets. Instead, try to spot one of its eight visible moons. Titan, the brightest of the moons, is within a reasonable magnitude to be resolved by the Celestron 130GT telescopes should you achieve enough magnification, however the rest will present a significant challenge due to their combined low brightness and small size.

Jupiter is consistently one of the brightest objects in the night sky, and as such is an optimal planet for visual observation. Its size and brightness make resolving surface details possible even with smaller telescopes. When conditions are right, four of Jupiter's 79 moons, the Galilean Moons, can be seen as well (these are Callisto, Europa, Ganymede, and Io).

While capable of shining with incredible brightness, Venus' atmosphere makes it difficult to resolve surface details no matter the magnification used.

With enough magnification, phenomenon such as Schroter's Effect, and Ashen Light can be observed. These are detailed in the Planetary subsection: About the Planets

At opposition, which happens about every 26 months, Mars can be one of the brightest objects in the night sky. At other times, however, it can be fickle and stubborn to observe.

Use a red filter when observing Mars to bring out additional surface details when brightness is sufficient.

Under clear, still, and dark skies, it is not incredibly difficult to find Uranus in the night sky, however due to its distance, the planet is difficult to observe consistently and with any detail. For reference, expect to need at least a magnification of 500x to begin to resolve any surface features.

Much like Uranus, this planet, while possible to see when conditions are right, presents a challenge to an observer with a less-than-substantial telescope. The same general guides for Uranus applies to Neptune as well, however expect to need at least a 900x magnification to find any detail.

Pluto, while not technically a planet anymore, deserves a spot on the list, even if it is just to acknowledge the difficulty required to observe it. Pluto's faintness means that it would be impossible to see with one of the 130mm Newtonian telescopes (at magnitudes of 14 or less, this planet falls outside of the scopes' limiting magnitude of 13).