A popular move nowadays. The old and classical choice is 6...Nb6, while 6...Nf6 is another modern try.
1. c2c4  1... g8f6  2. b1c3  2... e7e5  3. g1f3  3... b8c6  4. g2g3  4... d7d5  5. c4d5  5... f6d5  6. f1g2 