Below is pretty much everything I've added to the game and it certainly enhances it. Fristly, forget the cumbersome default launcher, get Content Manager!
https://acstuff.ru/app/
Weather/time of day: I use gbWSuite2.4 from Racedepartment. Combined with the Shader Patch: Dynamic Lights mod you can race at night with headlights.
https://www.racedepartment.com/downloads/good-and-bad-weather-suite.19484/
https://acstuff.ru/f/d/28-shaders-patch-dynamic-lights
HUD Apps: From RD I use Sidekick for compact race info and Initial D6 HUD 1.6 purely for the analogue gauges in older cars. I dont race online at the moment but HeliCorsa would be a good idea if you do.
Car mods: I get most from RaceDepartment. I wont list them all (just filter by ratings/dowloads) but I would absolutely recommend the Caterham Super7, Lola T70, NFS Class A and B packs, Primera BTCC, Williams FW26
The Aussie V8 Supercars mod is a must if you're a fan of the series.
http://v8scorsa.weebly.com/downloads.html
The only paymod I've ever bought is the RSS GT Pack and it is well worth it if you enjoy GT racecars
https://sellfy.com/racesimstudio
Tracks: Again I get most from RaceDepartment. User Lilski's tracks are top quality, his latest Watkins Glen is superb. The open road tracks by Phoenix77 like LA Canyons and Pacific Coast are wip but shaping up nicely too.
I've also downloaded almost all tracks listed here and they are very good quality, though all aren't finished yet. The Spa 1966 is stunning!
https://actrackrebootproject.wixsite.com/ac-track-re-boot