Cies Islands

Vigo is not a very interesting place. If you can make the trip to the island work while spending the night somewhere else, that's what I'd do. If staying elsewhere is impractical, just minimize the time you spend in Vigo. It's not awful; it's just that there are so many more interesting options.

There are (or were, as of 2016) at least two companies running boats over to the islands. You don't have to specify a return time when you buy your outbound ticket (which will have a fixed departure time), but you will have to return with the same company; no mixing allowed. Therefore, you need to consider the times of both outbound and return trips when making your initial choice.

There's a daily cap on the number of visitors to the islands, so if this is important to you, you might want to buy your ticket the evening before.

The beaches were sandy and not crowded at the time of my visit (probably in June). As an American, I didn't think simple, uncrowded, sandy beaches justified the amount of gushing online praise for the islands, but of course European beaches seldom look that nice. For myself, I regretted the time spent on that jaunt and the forced time spent in Vigo rather than in Santiago de Compostela, Pontevedra, A Coruna, Betanzos...

Baiona is a much nicer base than Vigo for the Cies islands. It is a good base to visit the entire area from Pontevedra to the border (esp. with a car) and has boats to the Cies islands.
I went to the Playa de Rodas on the main island in ~2007. Beautiful beach for sure, but not more so than some other beaches on the mainland, especially further north on the Costa da Morte (Carnota, Rostro, Traba, L'axe....). It's a good way to spend the day if you are travelling between Galicia and Portugal and aren't in a rush, but I would not go out of my way to visit in the context of a short visit to the region.