For an unique history lesson, check out the crumbling ruins of Banteay Kdei and try to imagine how it would have looked in its heyday.
You'll have sightseeing choices galore when visiting Siem Reap. Banteay Kdei is a good place to kick off here, but there are loads of other fine places to experience. Angkor Wat, Pub Street, and Angkor Night Market are close to the once-important structure and will fill in your day nicely.