Seurasaari is quite similar to Suomenlinna, only smaller. A straight circuit of the island on foot takes about an hour, but there are plenty of interesting old buildings to poke your nose into (most free, a few not), a couple of nice rustic cafes and even two nudist beaches (one for men, one for women), so it's easy to spend half a day here as well.
Most visitors opt to get there on bus 24 from the city, but my favorite way of getting to Seurasaari is on foot, since it's a nice, easy walk and there are a whole bunch of interesting places along the way. Here's a half-day itinerary I wrote a while back for Wikitravel:
And oh, don't try to cover both islands in one day, that's too much. If you have to pick only one, my guests have usually found Seurasaari to be the more interesting/"authentically Finnish" of the two. And while neither is particularly enjoyable if the weather is poor, Suomenlinna's exposed position means it's actively terrible if it's cold, windy and/or rainy.