From the main station (Gent Sint-Pieters) where you will arrive coming from Brussels, you can take any of these buses: 70 – 71 – 72 – 76 – 77 – 78, leaving from platform number 23 south side of the train station. Get off at the stop called “Lousbergbrug”. The bus trip takes about 15 min.
Your alternative is:
When you arrive in Gent-Sint-Pieters take any train direction Antwerpen or Eeklo and get off at Gent Dampoort. This takes about 7 min. From Dampoort station Castelnou is indeed about 1 km. Your train ticket from Brussels to Gent is valid for both the St-Pieters station and the Dampoort station (so no extra for Dampoort).
Besides the pubs popular with tourists (Keizershof, Trollenkelder, etc.), the following are worth trying:
’t Dreupelkot (=Waterhuis aan de Bierkant), Groentenmarkt 9
’t Galgenhuisje, Groentenmarkt 12
Café De Hel, Kraanlei 82
Hotsy Totsy, Hoogstraat 1
These cafés are completely different from one another: the first two are well known for beer and jenever (“Dutch Gin”). “De Hel” calls itself a “degustation salon” and serves mainly cocktails (they do have some beers) in a rather voluptuous orientalistic (not oriental) interior. In the sixties the Hotsy Totsy used to be favored by artists (writers, painters, sculptors, etc.). During the Genste Feesten (when you are there), there are plenty of music performances and cultural interviews organized in the Hotsy Totsy. For these activities you will need tickets (which are probably sold out by now).
If you are interested in the very local popular pubs where the people of the neighborhood come to watch football games over (plenty) of beers or play cards or darts, try “Willen is kunnen” in the St-Coletastraat (although decent it can be very lively there), or the cafés around the square “Ledebergplein”, especially on Sunday mornings when there is market in the square.
I hesitate to suggest “The Backdoor” in the St-Amandsstraat because the patron can be very, very grumpy (some might even call him impolite). He is a Britpop fan. As in an English pub, you have to get your drinks yourselves at the counter. Mainly beer and whiskey.
As for returning to the airport: pbuyse is right, on ***weekdays*** there is a direct train from Gent-Sint-Pieters to the airport every 15 minutes past the hour. (If you leave from Dampoort station you will have to change in Gent-Sint-Pieters).