Yeah, the traffic will be horrible and the forecast is iffy, but there’s a lot of doom and gloom merchants out today.
I would definitely avoid Land’s End, but as wallop said, Porthcurno is a great alternative: beautiful beach and the Minack Theatre if it’s dry, the Museum (best secret in the whole county) if it’s wet.
Eden will be rammed if it’s wet, and the roads will be busy whatever (though not summer-busy). If you’re just down for the weekend you’ll already be spending a fair bit of time in the car, so I would try to avoid too much driving around whilst you’re down here (but that’s just me).
But if you don’t mind getting wet, just get out and walk. The coast path will be quiet if the forecast is right, and the moors above St Ives will be empty, too. Men an Tol, Watch Croft, the Nine Maidens, Mulfra Quoit makes a nice loop for a walk. Bring a map!
Are you bringing a bike?