sounds like a good trip, and there's plenty to photograph in and around Riga, depending on what you'd like to photograph that is : )
the
Latvian Museum of Photography in Riga is definitely worth visiting, but when I visited they mentioned they might be moving the gallery, so worth checking their current location. Plenty of old equipment and a short history of photography in Latvia. They also host contemporary photography exhibitions, and I guess you could find out what they'll be showing on their page. There's also a camera/photography shop called
Baltijas Foto Service in the city (quite central) that sells plenty film (next door there's an Indian restaurant that makes excellent masala tea too : ) ).