Marlow - these are the perfect questions to start out with, I'll share my experience as an owner. Some responses though I do believe depend on the investor strategy as well as market type (luxury, working class or somewhere in between)
4 unit and less multifamily, and especially duplexes tend to use sale comparable, so forcing rent say 20% might be great for cash flow but may not actually affect your appraisal and potential sale value all that much. For example, I might put in new floors in my basement rental, add a washer dryer in each unit, and install other new cosmetic upgrades, which might get me 200 to 400 bucks more a month in rent (worth it here in jersey city) but my appraisal will probably not be significantly higher if I requested another.
Now, if I legally added a bathroom or bedroom, put an addition on a house, converted a garage into livable space, ripped out all the outdated utility infrastructure or other improvement like that (things that inspectors and appraisers work with on their reports) this would definitely add value. but...it all does come down to comps. its a tricky game im still trying to figure out myself as someone prepping to flip/improve a rental for my next deal.
Regarding cash flow increases, you could potentially rent out parking spaces or a garage if you have them available. The other option I think is obvious to refinance if you have a favorable potential gain from doing so. Another way I have increased my cash flow on a property is renting out by the room, where 2 or potentially 3 tenants live communally in one apartment. This also decreases non-payment risk, but can on the flip side add to headache if they don't get along at some point. Secondly, related to the prior strategy, I have increased cash flow on that same property of mine is renting out a bonus room to one tenant exclusively, which they utilized for an office space.
I hope some of this helps, and I'd be glad to learn of other individuals ways of increasing their cash flow!