Antigua is the most obvious and rewarding day trip from the capital — 45 km to the west by shuttle bus (about an hour), with its colonial architecture, volcanic backdrop, and excellent food. Most visitors find Antigua the better base for exploring Guatemala, but the capital is worth a night or two for its museums and energy.
The volcanoes visible from the city — Pacaya, Agua, Fuego, and Acatenango — can be accessed on day trips from Antigua rather than from the capital directly.