Discounts for multiple price field selections (e.g. select all five options and get $X off)
Maybe you could create the price set with 5 individual day options each at $50 and a 'Full week discount' at -$60 and then add javascript to hide the discount option and set it when all 5 days are selected.

As a javascript approach that is open to someone messing with it and selecting the discount option when they should not so you might want to add a check on the server side as well - depends a bit on your audience.