If you want to get away from the ubiquitous hip hop that dominates the bar-scene then this place may be for you; you can jump around like an idiot to indie till the early hours. It is probably not a place to pull though, you tend to look pretty dishevelled after a while. In my opinion this is the best club/club on Bar Street, I should know as I was there for two whole Summers.
This is the best bar by some distance, situated down the smaller bar street it is a sort of local's pub for tourists as you tend to return every night. The staff are very good and the drink selection is excellent (Guinness served as well as cocktails). Great 'people watching' seats outside and opposite a cigarette shop for emergencies. It's an Irish pub, serving Harvey Wallbangers, served by Swedes, populated by a variety of nations.....make sure you go.
Hire a moped and ride over to therma, it's usually hotter than my baths and probably yours unless you bathe in boiling water. It is however smelly, but exuding sulphur till you wash is a small price to pay. If the main area is not hot enough then you can always lie in the spring's source, which is too hot for most to consider (good if you want to show off!). Do be wary of what locals tell you, they are ruthless in their pursuit of your hard-earned euros, respect but don't trust.
The 'Old Town' is pretty at night, there are some good restaurants, look out for 'Never on Sundays', you will have to ask but it's worth it to get away from the crowd and have a more traditional Greek experience. Maybe hire a car and explore the island, there are several villages you can easily reach, visit Therma (natural hot springs). The 3-Island Cruises are good for families, opt for the bigger and more impressive boats.
Definitely worth a visit, the water is always very hot at the spring's source and the immediate area, and usually warm within the rest of the pool. You will smell of sulpher till you wash but this is a small price to pay for enjoying this natural bath. I recommend the late afternoon and early evening (and very early morning if you don't want to sleep!) as you appreciate the water temperature more. I spent two summers in Kos and was at therma three or four times a week...and apparently it's good for your skin: any excuse!