Can I apply festival prices to every product except a few?

No, you can exclude them.

1. Go to Plugins → Price Toggling.
2. Set Apply To to Selected Products (or By Category, By Brand or By Tag if the products share one).
3. Set Match to Exclude — all except these.
4. Add the products, or pick the category, brand or tag, you want to keep at full price.
5. Click Preview, then Activate Toggling Price.

Every other product that has a Toggling price gets its Sale price updated. A quick way to group the excluded items is to give them a tag like "no-sale" and exclude that tag.