Creating a Shop-wide sale

I'd love to see an option to setup a sale on all site-wide items for a time period.  I.e. 20% off Everything, or 20% off a category, or 20% off items that are cheaper than or more expensive than an amount.  I'd also love to see a way to temporarily turn off the cost of shipping for items, for example, turn set site-wide shipping to $0 for 2 days -- and then have it go back to the default, it'd be nice to make the sale amount able to be temporary as well.  Thanks for listening!