Riga is a nice place for visiting. Great old town, lots of sights to see, several interesting museums, and nice pubs with cheap beer. If you can, try to get a sightseeing tour around the old town - it should take about an hour or so and you'll hear and see plenty of interesting facts about the past and present of the city.
If you're in for something aviation-related, there's a museum right next to the airport, with several Soviet-era military & civil aircraft and other stuff.
There's also the town of Sigulda, about an hour and a half from Riga by train. There you can find the ruins of the Turaida castle, the largest cave in the Baltics, the folk song hill and I think you can even try bobsleighing there. The terrain's quite hilly and can offer some rather nice sights (especially in the autumn when the leaves on trees can create some nice colours).
Riga In Your Pocket is a relatively comprehensive guide to the city and can be found here:
In general, Riga is definetly worth visiting - I've been there myself on numerous times and hoping to go for a daytrip again this Christmas.
[Edited 2005-10-18 23:39:49]