Tourist board:
There are a number of events all year round in most cities, you just have to find them. Usually the local tourist board knows what's going on and put up events on their website. Just have a look every morning and put things that sounds cool on your notice board or print off the event's poster and put it in a place where everyone can see it.
Local magazines:
A lot of cities have magazines that keep you informed about the local cultural scene - the newest hottest bands, artists, galleries and gigs. Quite often they have a printed version but their websites are even more up to date and offer a bigger data base. And just like with the tourist board, check their events in the morning and pick something you think your guest might fancy and let them know about it. Who knows they might go and see someone in a small club that becomes the real deal a few months later...
Local pubs:
There are always events in your favourite pubs but quite often they don't make it onto the websites I've mentioned above, sometimes not even onto their own website. But their facebook or twitter pages might be better maintained. You can either like or follow them through your personal account or even better as your hostel's. That way you can easily share events and promotions. That does not only inform your guests about what's going on but also creates a positive relation ship with the venue and who knows, they might think about hostel discounts in the future, because let's face it, they are struggling too in the quiet season. Or simply pick up a flyer next time you're there..
Start your own events:
Of course there is the standard pub crawl almost every hostel has and it is an integral part of hostel life, because it is one of the easiest ways to bond. Getting buzzed and making out with total strangers from halfway around the world...But there are endless options. How about a games night where you dig out your Monopoly, the old Chess board and the Poker cards and save them from disappearing all the way in a pile of dust. Or have a potluck where everyone contributes something. It can be themed as in everyone brings something from their home country or has to invent a disgusting name for their dish.
Dig up the old dress up box and have an impromptu party, chuck paper and pens on the table and let them draw and paint, especially for events like Valentines Day or Easter. They generally enjoy anything children enjoy... bubbles, lego, arts and crafts, sometimes even jig saw puzzles...The important thing is to bring a little color into the cold and grey winter days and motivate your long and short term guests to do something beyond the same old same old every day. It creates a positive atmosphere that everyone will appreciate and keeps things fresh and exciting. And who knows, it might even influence your ratings on one of those booking websites...
