Great for partying/messy weekend away, not so much for sightseeing
Don't get me wrong I loved Riga. Nightlife is one of the best I've experienced. Enough to do during the day for 2/3 days (especially when you're sleeping through half of it from being out until 6am) but if you're going for a couples weekend then I can't really recommend it as you may get a bit bored after a while.
Insider Tip:If you're there for drinking then stay at the Naughty Squirrel Backpackers hostel. If you can't then go to the Aussie bar round the corner to meet some like-minded travellers and make sure you go to the Rock Cafe between midnight and 6AM!