What do you need? (1) Most want airspeed but for really low and slow angle of attack indicator would suffice, If you know RPM's of engine + angle of attack you pretty much know if you are climbing, descending, or more or less straight and level barring the effects of severe downdraft. (2) Turn and bank, next I want to know if my wings are level or in a turn and if it is coordinated. (3) Altitude If you know the height of terrain below you and subtract it from the altitude displayed you know how much room you have to screw up in. (4) Heading, am I going in the direction of steeper terrain or lower terrain or maybe just the direction I want to go.
Back when I flew for pure fun interesting things appeared below me, Moose appeared not so infrequently, People and people stuff of course there is always that, Military was sometimes funny ... well at least then it was, coming over a ridge and seeing an encampment with the full smorgasbord Tanks under camo and people in camo pointing and stuff, happened several times. I don't think they saw the little Champ coming on there equipment back then so surprise! Of course I flew with a sectional chart and an enclosed cockpit so learning the art to fold a chart so you can "roll it" while flying and keep the pertinent information near where you are at is a skill you should master but ... hard to imagine in open cockpit, bet more than one chart went with the breeze.
So (5) rudimentary map, for gods sake you only need to know where you are in relation to a few things and combine that with your heading information, you do not need to know how to get to Mars.
Likely (5) should be on a momentary switch with heading and altitude.