well, i see that others have given some useful tips to you! I agree with them - 'Pulkvedim neviens neraksta' or 'Cuba' are nice places to go.
'Cuba' is kind of 'salsa bar', very popular in Riga, they play salsa, funk, light jazz, the place is quite small, though. Yepp, I just remembered that entrance is for 21 year olds and over... Too bad!
'Pulkevedim neviens neraksta' is bigger, with several rooms and they play various music styles. I'm not sure about their age limitation.
Riga is full of young people, it's not only the capital of LV, but also the economic, financial and cultural centre. Most of young people study in Riga and after completing studies stay here.