Some of the numbers quoted here depend on you having the 200% login bonus. Of course, if you love GT5...
That said, the new Seasonals are amazing earners. I was playing last night, maxed out my cr, spent it all on a new RB2012 (that I didn't need) and I'd maxed out again before I went to bed (about 5 hours later). Honestly, the only cr problem you have now is what to spend it all on.
My personal fave is the 650pp event @ Grand Valley. 5 laps of fun in under 10 minutes for between 400,000 and 800,000, depending on your bonus level.
If you want an earner while you're doing something else, the new B spec are great, but La Sarthe is tricky without intervention. The earnings here are in the millions and there's a cool mil xp for your Bobs for a win too.