Pha That Luang, a giant golden Buddhist mound recognized as the most important national monument in Laos, is a straight-forward half-an-hour walk from Patuxai, Vientiane's Arc de Triomphe. You can see it from the outside, but it costs 5000 kip to see the gold stupa up close. Lower your expectations as the stupa looks nice enough but probably needs some upkeep.
The free temple next to it with a golden reclining Buddha has very interesting murals and locals sleeping and hanging out inside.
The other more impressive-looking building is rather empty.