If I had an unexpected day off at the moment I would definitely have a lie in as my job is very stressful right now and I am exhausted!
However, I always plan lazy days but end up rushing around doing stuff anyway. So I would go shopping (having a baby is the most fabulous excuse to b*y lots of stuff!), do some things around the house (with 60 hour working weeks I never get the time) and cook lots of nice things.