Mine was to work tax policy to incentivize companies to make all their packaging shiny and white, incentivize people to litter, and disincentivize everybody from recycling.
My friend's was to use a giant rocket to push the earth farther away from the sun
We might regret pushing out the earth's orbit when the present interglacial period ends. (Also, careful: to push the earth's orbit further out you don't want to be pushing the earth further out directly; that will just make a more elongated orbit. You need to push mostly along the earth's direction of travel.)
Make a little Dyson swarm (maybe somewhere further in than the orbit of Mercury) to reduce how much of the sun's output reaches us in a form that can get through our atmosphere.
Apply genetic engineering to our crops, our food animals, and (at least for people in hotter regions) ourselves, to cope happily with higher temperatures. (And gradually move in away from the coasts, and learn to be less worried by hurricanes.)
Painting roofs and roads and things white (an obvious extension of the bright-white-litter idea) has actually been not-so-crazily suggested. I don't know how it compares as a solution with turning them all into solar panels in order to reduce the amount of CO2 we need to make for energy generation.
Construct lots of gigantic heatsinks, attached to the earth but stretching up above the atmosphere.
Wouldn't the heatsinks need to have very high temperature conductivity? Do we have suitable materials today? Are we talking about mountain-masses of superconductors?