Karnak Temple is simply jaw dropping, and should not be missed under any circumstances. The temple is actually an enclosure containing a number of temples to Amun, and some parts of the eastern side of the complex may have been built as early as the 20th century B.C. It seems that almost every ruler who followed contributed to the site until it was a vast and spectacular collection of temples, halls, and statues. Now, since the entrance is on the western side, the general rule is that the farther you go into the complex, the older the surroundings become.
You'll enter through a massive pylon (it was never completed, but still stands 43m/141 ft. high) and find yourself fairly quickly in the famous hypostyle hall. There are 134 columns here, rising around you like the trees of a giant forest, dwarfing anything of human scale. If you've brought along a guide -- probably a good idea for this massive, and massively complex, site -- they'll stop you here and explain that the hall was planned by Amenhotep III (who built the older parts of Luxor Temple) but actually built by Seti I, with Ramses II (who contributed some spectacular additions to the Luxor Temple) adding relief work and decoration. They should point out the reliefs on the northern wall of the hall, which were done under Seti I and are of higher quality than those which were added by his successor. They should also add that the room was originally roofed, and that the columns and the walls were brightly painted.
From this point in the temple complex, you can continue east or you can head north. This second axis was added to the temple layout by Hatshepsut (1479-1456 B.C.). The east-west layout of the temple as it had developed to that point followed the track of the sun and represented celestial power, and this new direction, parallel to the Nile, symbolized terrestrial (and, by extension, royal) power.
Karnak can get very crowded. To avoid the squash, get here first thing in the morning. If you're going to go in the evening to catch the light, remember that because of the high walls, dusk comes 30 to 45 minutes earlier inside the complex than outside.
Karnak Sound & Light Show -- Gloriously over-the-top and cheesy, the sound-and-light show at Karnak is some of the best entertainment in Egypt. The 1-hour, heavily voiced-over show takes you through some of the history of the site and moves from the courtyard, through the hypostyle hall (which is absolutely magnificently lit and one of the very few examples of a modern improvement to an ancient site), and then to the sacred lake. Even if you don't like the loud music, it's really worth it for the walk through the temple at night. Make sure that you edge to the front of the crowd in the hall so that you get a good seat on the bleachers that are set up at the sacred lake. Admission LE50 ($9.10/£4.60). Winter shows 6:30, 7:45, 9, and 10:15pm; summer 8, 9:15, 10:30, and 11:45pm.