While what everyone writes about San Andres it very true: a lot of regional tourism, a lot of big hotels, not a great city – it does have a lot of great things too. It is a perfect place to relax on the beach, rent a buggy to explore the tiny island and go on an amazing boat/snorkel tour. You will see so many different fish, reefs, sea stars, and even manta-rays.
If you have more time, it does make sense to go to Providencia, it is quieter, beaches are less crowded and more laid back. However, it is expensive to get there and most importantly: very time-consuming. There are no direct flights from the mainland to Providencia and no good connections from San Andres, which means you need to stay 1 night in San Andres.
On the downside: there are very few cute & authentic B&B’s and there are no cheap hotels. The price/value of the hotels is not great. It is cheap however to get there by plane and LATAM has amazing price deals.