Some times modification may add value to the stock resale value (but not recently in current market satiation) like:
-swapping alloy wheels with original larger size with in the recommended size range from the manufacturer and by the same car maker brand.
-installing original body kit or trim body kit by the same car brandlike M-power for BMW or AMG for Mercedes or ABT, APR for VW and Audi or Wald and Veilside for Japaneses car brands
-interior trims like wood shape and carbon fiber trims
-Upgrades for audio system with original and branded systems with additional system options not over powered or loud systems like GPS, rear view cams, and AUX ports and sync with smart phones which is not standard in basic models.
Don't spend much as you would enjoy it and don't expect to earn it back on resale value.