El Calafate and Ushuaia are too far away for a weekend getaway, and days in August, albeit longer than in July, will still be very short. And, of course, both places will be very cold.
Iguazú for a weekend is tight but doable. Winters in the Northeast are very mild, around 15-25C during the day and ca. 10C during the night.
Mendoza, also tight but doable.
SCL: A weekend is enough for Santiago in my opinion, but there's no way you will manage to squeeze Valparaíso in such a short time period. Bear in mind flights to Chile now leave from EZE or EPA, which adds some time to your journey. It's quite cheap to get there, though -- JetSmart, which flies out of EPA, has had some interesting offers over the last few months.
Uruguay is the traditional weekend escapade for porteños. Colonia you can just visit in a few hours. Montevideo is nice and has enough things to do during 2 days. Plus with the ferry it's just a 2 hour voyage from downtown Buenos Aires to downtown Montevideo.
Otherwise you can arrange a trip to an estancia somewhere in Buenos Aires province. Red meat, wines, horseback-riding -- you know, the whole Gaucho experience. There are some very fine lodges out there.