It's definitely more challenging with an S trim. I suspect that potential buyers would be comparing your LEAF to SV and SL trims, to decide if their extra features are worth the extra cost. There aren't many comparables in terms of other EVs of that vintage, so you likely don't have to consider those...
I'm pretty sure a backup camera became a standard feature by the 2015 MY, but if not then you could add an after market backup camera. Same goes for an auto dimming rearview mirror.
Trying to retrofit the infotainment system from an SV/SL would be difficult and not worth the effort.
Some nicer wheels than the stock ones might help, but you likely wouldn't get your money back, unless you can find some used ones in decent shape for less than $500.
Honestly, other than perhaps having it professionally cleaned inside, and getting the exterior paint cut polished and waxed, I wouldn't do anything else.
However, if you have a better than average battery pack SOH, then try to educate the potential buyer on the paramount importance of the pack health and how that dictates range, so they understand that the bells and whistles on an SV or SL, when weighed against a healthier pack, aren't worth the the trade-off.
It wouldn't hurt to tell them about the merits of LeafSpy too, I would trust a seller who was transparent about the battery pack health and wanted me to be an informed buyer. Trust can go a long way, especially if the potential buyer is comparison shopping at car lots too.
You might also want to point out that the navigation is dated on SV/SL trims, that the infotainment center is likewise dated and just average, and that the BOSE stereo upgrade also isn't a whole lot better than on your S trim.
I bought the SV trim because I felt the hybrid heating (heat pump) would give me better range during the winter months. If I lived in a warmer climate, I would have purchased an S trim instead.
Vancouver, CA owner of a 2013 Ocean Blue SV + QC, purchased 01/2017 in WA
Zencar 12/20/24/30A L1/L2 portable EVSE
1-1/4" Curt #11396 hitch
After market, DIY LED DRLs
LeafSpy Pro + Konnwei KW902 ELM327 BT OBDII dongle
Loving my first BEV