What you want is low-cost, high impact changes.
IMO, cleaning is the #1 item on this list. A place that is free of odors (especially smoke), 100% cleaned top to bottom, scrubbed on your hands and knees, will attract the best quality tenants for the least amount of money. So clean until you could eat off the floors, the bathrooms scrubbed and shiny, all windows washed outside and inside, all carpets cleaned, all appliances shined with stainless polish. You get the idea.
Next would be paint, using trendy colors that appeal to a wide variety of individuals, both men and women. And it's imperative the actual paint job is done well. All holes spackled, and caulking should be done. The entire place should be sanded top to bottom, and 2 coats of paint applied. Also, all closets should be painted. 6 gallons of paint only costs $300 and is completely DIY.
Next on the list is taking good pictures, which has almost nothing to do with actually making improvements. Hiring a professional photographer or investing in a decent camera shows people a level of profession that is hard to come by for many landlords who won't bother. Also, nowadays where so much marketing is done online, having good pictures helps tremendously with advertising.
An uncommon one worth mentioning - closets. Remodeling closets is incredibly easy, DIYable, and can be a real selling point to new tenants. Having custom shoe racks, areas for jewelry, and tons of storage is very appealing.
Next is staging the apartment or house. Simply putting in furniture to make the place look good goes a long way. It helps people envision what their lives will be like.
In summary:
1. Cleaning ($100)
2. Paint ($300)
3. Pictures ($150, maybe more)
4. Closets (around $200 for all materials)
5. Staging (anywhere from $500-$3000)
Refinishing hardwood floors (About $3500 to be done professionally. They're quite worn/scratched in the living room but fine in the bedrooms.)
Completely unnecessary, IMO. If you spend a day scrubbing those floors chances are they'll look almost as good as a professional refinish. Then, if there's still major nicks and scratches, get a cool area rug.
Installing a natural gas fireplace insert in the existing, standard chimney (between $800 and $2000 including installation, and depending on what kind of fireplace we buy)
I would not do this because it poses a liability risk for gas leaks and such.
New front door (existing is "retro" and needs refinishing)
Painting can go a long way without the need to buy a brand new door and have it professionally installed.