We have units in both high rent and lower rent areas... some get ceramic tile flooring, pedestal sinks, bead board, central a/h and two tone paint. Others get peel-n- stick tile, a thorough cleaning, new ac window unit and a fresh coat of semi-gloss white on woodwork and walls :-)
Like others have already said... it really depends on the local market and what other landlords are offering at each price point. We try to make ours just a little nicer than the competition--anything more would be a waste of profit 8)
In general? Paint is cheap and always makes things look fresh... cabinets, walls, ceilings. We do semi-gloss in all rentals. Two tone paint is more labor intensive but is a nice upgrade that doesn't cost much. New hardware on kitchen and bathroom cabinets is also cost effective and looks great. New light fixtures can be quick and fairly inexpensive. Ceiling fans are great in our area-- we buy them at Wally World for about $40 each and install ourselves.
Personally, I *hate* carpet in rentals... hate cleaning it, double hate replacing it-- so we get rid of it asap in all our units-- ceramic tile if the rents will support it, refinish hardwoods if they're hiding underneath old carpet, sometimes even laminate (not the cheapest stuff... doesn't hold up to water). We're about to try some peel-n-stick vinyl plank flooring that runs about $1 a square foot and looks really nice. We'll see how it holds up :D