Someone has made a Tony Hawk's Underground 2 mod that allows you to skate Grand Theft Auto: San Andreas' East Los Santos locale like it's a skateboard game. Sure, you can BMX in GTA: San Andreas without the need of any mod, but it's not quite the same as skateboarding, is it?
Getting it all to work is pretty simple. You'll want to something called THUGPro. Technically, that's a mod-extension to THUG 2, but in practice, it's a community project that allows everyone to add levels and features from any Tony Hawk's game. People use it to create levels from other games, too, as you can see from PunkyDTH's LS East map. If you'd like some directions on how to it all, I'll point you in the direction of this nifty Reddit thread.
If you'd like to see it in action, Reddit JFedzor put together a video called "If GTA San Andreas was made by Neversoft". The THUGPro player even has a CJ skin, and kicks off the video with his classic catchphrase that's become a meme – I reckon it adds a nice touch.
If you'd like to see the video, you can catch it down below:
If you'd like to play some new Tony Hawks, the new PC games if you're curious about what else you should try.