Cruise control was my first thought. Cruise is useless for a lot of driving on the East Coast, but it can really make a difference on long trips with less congestion.
Air conditioning might be #1. In my twenties, I drove all over the U.S. in a Slant-Six Duster with three on the floor and power nothing. On a trip from Mississippi to Lake Tahoe, I rolled through Memphis on a morning so humid I could stare right at the sun rising red over the city. It was about 100 by Little Rock. I opened the under-dash vents and the windows, took my shirt off and draped it over my shoulder to block the sun, and sipped on Gatorade all day. I think Jigg's Smokehouse in Western Oklahoma was closed by the time I got there, but the thermometer on their porch was still reading in the 90s in the late evening. The West and Southwest were a little more forgiving. There, I'd be wiping away salt instead of sweat.
I miss that car--still dream about it now and then. But when I finally got rid of it for an 11-year-old Caprice with A/C and cruise, it was like a revelation. I could drive 8 or 12 hours and not feel like I had been stacking hay bales all that time.