Stupa of King Norodom Suramarit
Directly south of the Royal Palace in Phnom Penh is the Silver Pagoda. The official temple of the King of Cambodia, and a popular tourist attraction.
Like many temples in Southeast Asia, everything is ornately decorated. This was basically the very first destination I visited in Cambodia. Hitting up Phnom Penh before Siam Reap/Angkor Wat has the benefit that it's more of a build up to increasingly grand temple complexes.