I'd actually go the other way.

If you land in Narita at 6:40, you can reasonably well be at Tokyo Station at 9am.

Tomorrow there is a Kodama that would go to Odawara departure 9:33, it will not reach full speed to Shinyokohama, but to Odawara it will get close.
If you buy a return ticket from Tokyo Station you could easily catch the 10:14 Kodama back to Tokyo Station at 10:48 bringing you back to Narita not much later than 12.

There could even be time to throw in a quick visit to Odawara Castle (not that exciting, modern reconstruction but makes for a few nice pictures) before catching an 11:44 Kodama with arrival in Tokyo at 12:18. Getting you back to Narita before 2pm.

It would set you back a bit less than 10,000 yen for the trains.