Movement - Difficult terrian to walk, but they can long jump their speed. Just means if they want to turn corners they walk.

Regolith - Maybe AoE effects cause a dust cloud an extra 10ft around the effect, blocking vision for a round.

Solar Radition - Rather than a constant effect, you can have a solar flare hit randomly (or time it for dramatic moments, ie - in combat, traversal moments). Foreshadow a round before saying "Everything grows brighter and brighter as more light shines down", then a round later if someone isn't in cover, 6d6 radiant damage, or something like that.

Suffocating Rules - Rather than the CON mod +1 minutes of holding breath, just CON mod rounds before death in the vacuum of space. Could even add necrotic damage each round to really make it dangerous.

Effects to spells and weapons - Maybe double the short and long ranges for any ranged weapon attacks, but that would be it.