Catching a flight at 6:25am. I understand that there is no train or airport bus to get us to the airport to catch that flight. We are 6 passengers, no luggage. A long day trip returning late night.

Please recommend modes of transportation other than a taxi, if available.

Thanks

if you stay at a decent hotel, they can arrange a taxi at that time for you ahead of time.
shuttle services also must be booked in advance.
i have no experience catching flights at that time;sorry.

There's http://www.ShuttleDirect.com & AeroBús @ http://www.bcn.es/turisme/castella/turisme/mobilitat/ser_tra_hom11.htm

Good luck! Saludos, BarcelonaMan