I would like to ask, is it possible to do a day trip to Riga from Stockholm?
I found inexpensive flights by Air Baltic for 90 euro from Arlanda at 8.30 in the morning and return at 19.00 or 23.30 from Riga.
Riga has a small city centre. All tourists are there and most of the "sense" and energy is there. There isn't much around it and a ride from the airport gets you there. If you can come up with something you want to see, it won't be far from it and one day will be enough. Of course, if you're not planning to drink in the evening and have "adventures".
Maybe the only exception is the sea-side city of Jurmala, getting there is 30 minutes on a train.
I'd recommend using some guide like InYourPocket (they have PDFs on their site and you'll find the printed version in many local hotels) to find something interesting.
While I agree with Mikhail's answer, I would like to add more information.
First of all, AirBaltic is known to be very punctual, so you'll probably arrive in Riga without any major delays. The flight takes roughly one hour, the airport is small and easy to find your way around, then by public transport the journey to the center takes about half an hour, here you can find more details about that, but you could also opt for taxi to save time. Especially if you're visiting from Sweden, you'll notice that it's a fairly cheap place, that also concerns the taxi rates.
Riga has a small and pretty old town. In my opinion three hours is enough to enjoy the major sights and walk around the city, which would even leave you time for an enjoyable lunch before the walk and a coffee break at the end, as well as some quick shopping.