Empty cart page
When customer empties cart, or just in the empty cart page (same). Present customer with a list of products. Recently viewed, popular products from categories, etc. That the customer can easily press add to cart and they will jump into the cart. This should also be standard for when a customer just added a product into the cart. It will significantly increase add-on sales. Amazon use this very effectively.
When product is in cart, from redirect customer to cart after add to cart. Present related / customers also bought products under the product added to the cart.