A good several mentions of what to do in this thread already. Have a picnic! Picnics are always great.

My bessst friend and I do not spend days out. When I visit her, we spend our time indoors watching horror films, bad slasher films or Metalocalypse, shredding eachothers' Wreck This Journal, playing Silent Hill, watching giant squid and deep sea creature documentaries, making food, smoking hookah, hailing Satan and talking trout about everyone we went to high school with whose lives are worse than ours. We exit the comfort of indoors only to walk our dogs and to get cupcakes.