After “Whirlwind Weasel’s” Flip Effect has resolved, the effects of already face-up Spell and Trap Cards, like “Ultimate Offering”, can still be activated. The effects of Spell and Trap Cards that activate when the card is destroyed, such as “Blast with Chain” or “Ojamagic” will also still be activated. (In other words, you cannot activate a card, but you can activate an effect.)