There are Clipper machines at "virtual OAK station"
- Board the BART-OAK cable car*. There will be a conspicuous lack of fare-gates or entry controls. You will fear this is a proof-of-payment system. It's not. Don't worry about it.
- Exit at Coliseum Platform 3.
- There is nowhere to go except through fare-gates or back to OAK!
- Buy your Clipper card(s) right there.
If you intend onward travel on BART, tap through the fare-gates, and you are now in the paid area of BART. Where to go next will be obvious. Your card will be encoded as having entered the BART system at OAK Airport.
If you do not want to ride BART today, buy your Clipper tickets and hop right back on the cable car for a free ride back to OAK. This is legit. It's quite fast, and you could do it while your companions collect luggage.
How does that work? BART is a variable-fare system, meaning you must card-in when you enter, and card-out when you leave. However, the OAKland Airport cable-car is a simple 2-station shuttle, and for design-geometry reasons, the Coliseum end empties only into the paid area of the BART station at platform 3. There's no way to get landside from platform 3. So BART displaced the OAK fare-gates to the Coliseum end. Now they do not need to staff the Airport station.
Since the fare-gates are there, so are the ticket/Clipper sales machines.
Joyriding the BART system proper
BART has an "excursion fare" intended for joyriding the BART system by entering and exiting the same station. Normally this is overpriced, but from the Airport station it's the cheapest fare to anywhere, at $6.
So if you want to joyride BART proper, you can enter the faregates and take the normal BART trains. Complete your joyride within 3 hours or you'll have to see the agent and accept a stern look.
The Dublin line is easiest to access and it's #2 for scenery.
The Fremont line is just more of what you see on the boring part of the Dublin line.
The scenery king is the Pittsburg/Bay Point line, but you must change trains to reach it (this isn't too bad though).
San Francisco line is dead last for scenery as it's all underground west of Oakland. The cool spots east of Embarcadero are the railyards and the Imperial Walkers (really) at West Oakland and the black coolness of the Transbay Tube.
Richmond Line, only the far half is scenic, and it's a long drag underground through downtowns Oakland and Berkeley to get to it.
The short trip is a bounce to East Dublin and back. The longer trip is to Embarcadero then Bay Point then Macarthur then Coliseum, but that's ambitious if you only have 3 hours to kill.
* Yeah. It's really a cable car. Not quaint, but modernistic because BART. It doesn't have a grip or gripman, and the cable is permanently bolted to the car, unlike the system across the bay. The cable stops and starts to move the car. You can watch the sheaves turn at the OAK end; that's the powerhouse.