Once in the city you can do a couple different things to get to Albrook; take a taxi, the most expensive way, take a Diablo Rojo (red devil) bus, the local transport, the metro bus, which doesn't run to Albrook that frequently, or the metro. This is all from the fish market. You walk under the bridge, you'll know once you're there and can see this, past one street and then there will be the main bus terminal.
Here you can buy either the regular metro card which works for the metro buses and subway or you can buy the Rapi Pass which works for everything. If you are doing any travel, including any long distance travel through Albrook, get the Rapi Pass! You only need one card for your group or couple, just make sure the person buying has their passport on them!!!
Getting from the Casco Viejo area to Albrook is easiest via the subway metro, not the bus. The buses run very infrequently. The subway is only 10 cents more and it's only one stop!
Alright, now you have all your cards and you're exploring, but what bus to take back to Casco Viejo? There aren't any buses that say this so look for the buses going to Panama Viejo. From Albrook station there is a stop just a little down from the fish market. Just keep your eyes open and press the 'stop' button to make sure the driver stops there. The same goes for if you're going into town. Just take a Panama Viejo bus back the other way and get out where you want. We actually walked back along the water and it was a beautiful stroll back!
Hostels and hostels should also be able to help with some of this. Ours tried but left out some valuable details so I hope this will make you're stay just a little easier! In the end, with getting the two cards, we still spent much less between the two of us than if we'd done taxis or the sight seeing bus.
Safe and happy travels!
