Hello Lucy,
Don't worry, if you look for, you will find good night atmosphere.. I've travelled some times to Punta Cana and I've noticed in my last trip (NOvember/00) there are much more things than two years ago (of course, Punta Cana is not Sosua, Boca Chica, or Santo Domingo, but you can do something....)
First thing: You would have to take a taxi. It's not cheap, but if you make a little group, transportation would be much more cheaper (logical deduction).
I think that Sirenis is in the north of Bavaro, that is, near Riu complex and so on.
I suppose your hotel will have their own disco, nice to start the night, but not enough.
Good places (for me):
- PACHA: Disco at Riu resort. Big, many people (tourists and dominicans), and mixed music (not only american music, also merengue, and "holiday songs")... Tipycal holiday disco.
- EL RINCON: Different oppinions about it. Only tropical music (merengue, bachata and salsa). Many dominicans in a little dark place.. I like it. It's in Plaza Bavaro.
- HURANCAN CAFE: Just besides Melia resort. It's a bar with music at night, but you can dance at the beach on the sound... Many european music,..
I know there are other places in Punta Cana, but the truth is that you have to work a bit to find them (talking to dominicans is the best way to find night life....).