Narita-san or Sawara on way to Airport?

On my last day, I really have no plans and my flight does not leave until 18:30. Hotel I am departing from is in Asakusa. Narita and Sawara are both near Narita airport and are destinations I would like to see. Would it be realistic to try to visit one or both of those and still make an international flight that leaves in the evening? Is there somewhere I could store our 3 suitcases for the day near the airport or Narita city?

I've done that trip having an evening flight. I was able to check my luggage at Narita Airport in the morning. It might depend on your airline but I didn't have any problem as the desk was open. I then took the Keisei local to Narita, walked over to the JR Station and took the JR to Sawara. It takes 30 minutes on the train to Sawara and there's about a departure every hour in each direction.

Turn left out of Sawara Station and follow the tracks until you come to a canal, turn right and follow the canal to the old section of town and the Ito Museum. If this is the only place you're going, you can cover it in a morning. You could then come back to Narita Station, turn left out of the east exit and follow Omotesando down to the temple complex.

If, however, you want to see the Katori Shrine in Sawara, you won't have time to see Shinshoji in Narita. You can probably take a taxi to the shrine from Sawara to the shrine to save time, but returning taxis are rare, and it's a long walk back to Sawara or over the hill to Katori Station. (It's a nice country road walk to Katori Station, though).

I checked out Sawara and Katori, walking to Katori shrine and then from the shrine to Katori Station, and it took about 5 hours total from Narita Airport and back with a couple of hours to waste in the airport (also a 6:30 PM flight).

I forgot to mention that Narita Airport does have a luggage check if you can't check in early.

https://www.narita-airport.jp/en/service/svc_06

Sawara and Katori jinja are definitely very nice places. If you get started early enough in your day you should have sufficient time before your flight.
Just consider taxi to the shrine as it is on a steep hill a bit outside of the town.

http://bicycletraveljapan.blogspot.nl/2016/10/toride-to-kasumigaura-sa...

Enjoy your trip to Japan!

Thanks for the answers. I think we will try to at least one of those on the way out.