You'll have to watch how old your gas is getting (you don't say how far you will drive in the summer) which can be as easy as keeping your most recent receipt.
It's less of an issue with modern, fuel injected cars. The fuel system is totally sealed, so there's none of the "light fractions evaporating" problem that older cars had.
SeaFoam or StaBil is certainly useful, but it's not as required as it once was.
That said, if you can find ethanol free gas, it's going to last longer.
You might want to get a solar battery charger that can lay on your dash and plug into the cig.lighter.
Some cars turn the lighter plug off with the ignition (mostly Japanese cars), others leave it on. That's only useful if the lighter plug is live with the key off. But, yes, they're a nice addition for an infrequently driven car.