If you can't take the whole weekend - you can join BCI for the ride to Oceanside & train back!
Start at 8:42 with the 'tourists' from the tracks when the 857 train rolls in - Ride all the way to Carlsbad or stop in Oceanside & catch the 665 train at 1:24 to be back in Irvine by 2:17 or the 858 @ 2:51 to be back by 3:48. All day costs $10 for a weekend day pass or one way is $5.50 for "1/2 price" seniors 65+.
Register in advance with the USMC! (or else you will have to ride the freeway shoulder between Las Pulgas Rd & Oceanside - Legal, safer and shorter, but not as scenic as Camp Pendleton)