I activated a pack last Saturday night and didn't pitch it in until later in the day on Sunday. Brew has been fermenting just fine. You can leave it out in a cool room (within the yeast's posted temperature range would be fine)... Just sanitize the packet before you open and pitch. So, basically, business as usual.
On Tap: Caramel Ale, Mocha Porter II, MO SMaSH IPA Waiting/Carbonating: 12.5% Wee Honey II, 8.9% Old Ale, English Brown Ale, Lickah ESB, Mocha Porter II Fermenting K1: K2: Epic mead K3: TripSix On Deck: Caramel Ale Aging:mead Mead [bottled]:Oaked Wildflower Traditional, Mocha Madness, Blackberry Melomel, maple wine
You most definitely can use the yeast a while after it has been activated. All activation does is dump a little wort and nutrient into the yeast to kick start it. If you don't use it, it will just be dormant again. Not a big deal. Just do your starter like normal.