A great way to fill in gaps in your calendar is to sell last-minute availability at a discounted rate.
Just create a second Rate Type to get started.
Go to Prices at the top of the page, then Rate Types > New Rate Type
Give your Rate Type a name and a description
If you want to use the same prices as your standard rate type and discount them by an amount or percentage, tick Inherits prices from another Rate Type
Select the correct Rate Type from the drop down list
On the right hand side, tick Last Minute Rate and specify the number of days before arrival that the rate is available
Tick Modify all prices by a given amount or percentage?
Select Decrease by, and enter the amount or percentage
Click Save Rate Type and Start Loading Prices
Let's see that in action: