Travelers: what do you do during your "downtime"?

From looking at the [spreadsheet that was meticulously kept by rixie](http://www.reddit.com/r/travel/comments/muofe/finance_charts_for_my_15_year_roundtheworld_trip/) I noticed that many days had no expenses other than food and accommodation, often not even public transit. I'm sure there'd be plenty of sitting on a beach just chilling, or perhaps walking tours of nearby admission-free sites, but what else?

I am planning an extended trip myself and I figured I'd have to budget some "slack time" as well since I'd probably not go touring, hiking, climbing, skiing, snorkeling, etc. every single day, plus the weather can sometimes be uncooperative. What do you tend to do when wanting to take it slow for a day, or if it's raining/snowing and you'd rather not go outside?