One night layover in Helsinki... about 12 hours

Don't miss the water front area (where they have the open air market) and that huge white cathedral (forget name) that's just off of that area. The russian orthodox church (this huge red church) is also another Helsinki stand-out. Also, the circular "rock" church (also, forget name off-hand). It's a round building literally built into the rocks, so the interior walls consist of all of the huge rocks/boulders you see outside. Pretty phenomenal building/very peaceful/simple. Very different from, say, the ornate churches/cathedrals that you would see in, say, France, Germany or Austria. Get Rick's Sacndinavia guidebook. In all of his outstaning guidebooks, he always has VERY easy to understand, helpful itinerraries, based on how much time one has. And he lists the "musts" (vs. the "secondary" sights) of any location, so this should help you to see Helsinki's top attractions. I didn't have long in Helsinki myself; had ONE full day (I had taken the popular over-night ferry from Stockholm; arrived early AM, had all day to wander around, departed for return trip to Stockholm later that night. But thought it was enough to get in most of the highlights, thanks to Rick's easy tips. It's a pretty, enjoyable city; enjoy.

Does that 12 hours include your time for sleeping? If so - sorry to say - I would just sleep. Helsinki needs at least a few hours of energetic sightseeing during daylight hours to make it worth the effort.

We have approximately, a 24 hour layover with about 10-12 hours for sight seeing.
I have booked a room Sokos Hotel Albert in the heart of the "historic and cultural" center. Or so it says!! The rates and the reviews seemed to be favorable.

10-12 hours for sightseeing is enough to get the highlights of Helsinki (though it is worth more IMO). As the other poster said, be sure to see the harbor area, the Lutheran Cathedral, the Russian church and the Church in the Rock. The train station is an important architectural landmark (Eliel Saarinen) and there are two museums nearby the station worth a visit if you have time. For a quick but tasty lunch, stop in the market hall on the harbor and eat at one of the food stalls, you can get authentic Finnish foods. The cafe in the glass pavillion on the main boulevard is also wonderful for a coffee/treat. On a nice day that park is packed with people.

the downtown arae is a pretty small place you can walk it. i went to an island only accessble by small boat for dinner where you can get a pretty good city and harbor view, good raindeer meat dinner. you can have some beer in a bar outdoor if you get warm and long days. it's not too far from the airport, i recall driving about less than 1 hr. road is wide open very similar to the US, and parking downtown is not too bad.