Interior lighting is the easiest and cheapest change to make that will make the interior feel fresh. You can get LED lights to put in the dome, reading, and console/cupholder lights to make the interior brighter and colored to your liking.
For example, I got a bright white LED for my dome light, now my whole interior is brightly lit up when the dome is on, its easily twice as bright as the old incandescent bulb. The passenger reading light is also a bright white LED so passengers can read or look for things at night without bothering my vision. Whereas the drivers reading light is a red LED light, I put that back in when I was working security, so I could still read and write reports, but not ruin my ability to see in the dark.
My tail/ turn and reverse lights are LED and it makes them much brighter, but more importantly, they haven't burnt out yet. Until I install high power bed lighting, the LED bedlights make it easy to find things in my toolbox at night. In total I spent about $130 on all my lighting (not including headlights) and it feels like a newer truck that can compete with these new ones in terms of visibility in the day.