Jupiter is the object that converts people. It is bright enough to cut through city light, big enough to show detail in a small instrument, and it changes visibly over an hour, which almost nothing else in the sky does.
What the four moons do
The Galilean moons are the easiest thing in a telescope after the moon itself: four points of light in a line either side of the planet, visible in binoculars and obvious in anything larger. They move fast enough that their arrangement changes between one night and the next, and over two hours you can watch one emerge from behind the disc. A moon casting a shadow on the cloud tops is visible in a 130mm instrument on a steady night.
What the disc shows
At 76mm Jupiter is a small bright oval with two darker bands across it. At 114mm those bands acquire structure. At 130mm and up, in steady air, festoons and spots appear along the band edges and the Great Red Spot becomes findable when it faces us, which is about a third of the time given the planet's ten hour rotation.
Venus, which is different in an instructive way
Venus is far brighter and shows no detail at all: a featureless white crescent or gibbous disc, because what you are seeing is the top of an unbroken cloud deck. Its value is that the phases are obvious in any telescope in this index, and they are the observation that settled the question of what orbits what.
Questions people ask about telescope
Can you see Jupiter with a telescope?
Easily, in every telescope in this index and in ordinary binoculars. The four bright moons are visible at almost any magnification and the cloud bands appear from about 60mm of aperture upwards.
What magnification do I need for Jupiter?
Around 100x to 150x is where most of the detail appears. More than that usually just enlarges atmospheric blur.
What does Venus look like through a telescope?
A brilliant white crescent or half disc with no surface detail, because you are looking at the top of its clouds. The phase is the whole point of the view.