If you don't have it crown molding or even chair rails are a frugal way for an upgrade and adds ton of charm and some value to the unit. Or have them choose from a list of what you can offer
Renewal monthly parties are fun in an active community. You can have a “private” party for everyone who expires that month. It doesn’t have to be fancy but make it just about “March” renewals. At that time you can ask for feedback on what your residents think are cool incentives. You would be surprised of how inexpensive their recommendations may be.
I've always loved the allowance idea; we do pre-renewal inspections of all apartments, that is a great time to discuss.... "if we offered you $200 of updates/upgrades, what would you choose?" (and be ready with some pre-approved options, since you don't want every apartment looking completely different....) - but smaller fixtures, accent walls, kitchen tile, bathroom tile, new mirror, etc., anything small but impactful would be good. Does take some org and follow-through. Good luck!