San Andrés is Colombia's most accessible Caribbean island escape — closer to Nicaragua geographically than to the mainland, but well connected by direct flights from major Colombian cities, which makes it one of the more realistic add-ons to a Guatapé-centered trip.
The connection logistics
Both Medellín and Bogotá have direct flights to San Andrés, typically around 2 hours. There's no need to backtrack through Bogotá if you're flying from Medellín — check both Avianca and Wingo, which both serve the route, for the better fare on your dates.
Suggested split
A clean structure: 4–5 days in Medellín (including the Guatapé day trip), then fly directly to San Andrés for 3–4 days of beach time, snorkeling, and the famous seven-colors sea. This keeps travel days to a minimum — just one flight leg — while giving you both a mountain-lake experience and a full Caribbean island stop.
What San Andrés adds
Turquoise water, one of the most swimmable Caribbean beaches in Colombia, and an easy day-trip option to Johnny Cay. It's more developed and tourist-friendly than Providencia, making it the easier of the two islands to bolt onto a shorter trip.
Budget note
San Andrés carries an import-driven cost premium common to island destinations — expect food and some activities to run higher than mainland prices — but it remains more affordable and easier to reach than Providencia.