Batteries: Homey don't play dat.
....and my eyes are terrible, so the answer is scope. Specifically, my two favorites which have been within my realistic budget are the Nikon Omega / Slughunter 1.65-5x36mm (the most versatile scope ever made), and the Burris MTAC 1.5-6x40mm (currently being worn by my only AR).
Honorable mention to the Burris MTAC 1-4x24mm and the discontinued Trijicon Accupoint 1.25-4x24 1" (not the newer 1-4x24 30mm, which has crappy eye relief). Sadly, I haven't been able to afford to try any of the great new crop of tac scopes in the last 5 years - whether IOR, Nightforce, or you name it. The Burris is a good value in that you don't need batteries to use the awesome central dot reticle (unlike a lot of makers), which is why these still work with my down-with-batteries paradigm.
But I like / want the RMR-DI for a crossbow, a 10mm handgun, a .30-30 lever gun, and a .22 magnum pump (rossi 59), although I haven't completed my acquisitions in this set yet - need 3 more.