You will have plenty of choices I am afraid...
For a nice "before" I suggest one of these (assuming your weekend is during summer):
- the bar of the Opera, there are free jazz concerts at 7:00 and 8:15 pm and the atmosphere is great (you may share a table as you are alone):
opera-lyon.com/Peristyle-Concert-de-Jazz-et-…
- or enjoy a drink at sunset on the Rhône left bank, on board of one of the boats there or on a terrace along the river.
Regarding night bars for an "after", you should stick to Terreaux area (1st arrondissement); you may like:
- Café 203 (9 rue du Garet), it is open until 2:00am
- Le Voxx (1 rue d'Algérie), open until 3:00am
- La Pêcherie, 1 rue de la Platière, open until 3:00am
- one of the Ninkasi bars; they make their own beer and have several bars in town (clinets are mostly students, may be not exactly your age range, but easy to mingle), including a concert venue in Gerland:
http://ninkasi.fr/
A little trendier ?
- à KGB if you like vodka : http://www.akgb.fr/
- la Marquise (an other boat on the Rhône bank) : http://marquiseclub.free.fr/
A spanish bar with tapas ?
- Café Leone, 8 rue de la monnaie (2è)
There are also a few gay places that are great.
Last but not least, you may also try an english/irish pub in order to meet english speaking guys : such as the Wallace, or any of the pubs along rue sainte Catherine (1er), or an australian café:
http://www.wallacebarlyon.com/home.html
http://www.ayersrockcafe.com/