It's difficult this one as there are so many things... I suppose it depends on the company and mood...
However.. for now.. a good night in would consist of a huge mug of low fat choccie horlicks, snuggled up under me duvet with me leccy blanky on watching the latest period drama or murder mystery on telly.. sad innit!
Alternatives:
Girly Night In: Get the girls round with wine, chinese takeaway and copious amounts of choccies & have an Ann Summers Party!
Lads Night: Get the lads round and watch some bike racing whilst munching through some chilli and spuds all washed down with whatever they bring round to drink!
Better yet.... GO OUT!!!