Moon, planets---then the Messier catalog.
For guys: A sexy girl living in the apartment just a hundred metres away from where u live.
For girls: A handsome guy strutting half naked around the house in his apartment a hundred metres away from where u live.
With a fairly good amateur telescope, I would say the moon. Followed by the planets. I saw the rings of Saturn one night so clearly it looked like you could touch them.
If you have the coin for a telescope with a tracking device to keep it moving with the earth, err opposite of the earth that is, and can attach a camera to it, you could do time exposures and capture more dim objects.
Great hobby, with I had the time.
#1 is the Moon. Everyone is impressed by the Moon in a telescope, because it shows so much detail.
#2 is Saturn. Even a really small telescope can show the rings and it never fails to impress people.
#3 is Jupiter.
#4, is star clusters. At least the brighter ones.
#5 is a few bright nebulae.
Forget smaller, dimmer stuff, like galaxies.
As several people have said, you need to clarify what you mean by a "simple telescope." For me, that's at least a 6" Dobsonian, which costs about $300. Given that, my favourite objects would be:
? The Moon
? Star clusters, especially the Double Cluster in Perseus
? Emission nebulae, especially the Orion Nebula and the Swan Nebula
? Double and multiple galaxies: M31/M32/M110 in Andromeda, M81/M82 in Ursa Major, M51/NGC5195 in Canes Venatici
? The variable star SS Cygni
? And hundreds more!
To make Constellations, then observing the moon & finally the planets. (but i will try not to give much time to make constellations as gaileo was blind before death, due to obseving stars for long time)
The moon, anything else is a waste of time looking at through a simple, low power telescope.