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I have a American passport, but I've lived in Panama over 5 1/2 years now.  Americans and Canadians have the same requirements for visas unlike Brazil where we have different ones, because of US policies toward Brazilians. 
You won't have any problems coming in through an airport with needing a return flight, however when you renew your tourist visa by the time needed and you're taking a bus to Costa Rica via Paso Canoas, you will need to buy a round trip ticket even if you don't use the round trip.  One way costs about $14-$25 depending on your bus ticket and line of choice. 
Panaline or Ticabus are your only choices for round trips from Panama to Costa Rica, and you can purchase those at the Albrook Mall bus terminals/open mall side of Albrook mall.  Keep in mind it will be a 13+ hour bus trip with one stop at the frontera that includes waiting in lines for a 2 hour Panama/Costa Rica customs checkout.  You get another 25 min. break in the trip for a food/bathroom break at an outdoor cafeteria/truck stop designated by the busline. 
Entering Panama will be just as much fun except for another line to get in to pay a $5 tourist fee, after which you stand back in line for entrance into Panama.  As opposed to the airport where you pay your $5 tourism fee as you enter with your passport.  Also, you will be asked at the frontera to pay $1 for a sticker that has log numbers, but no log that will be placed on your passport page to be stamped before you enter the country.  Sometimes the people with stickers are there waiting for the tourists and other times not.  They seem to come around only during the scheduled bus arrivals.
Keep in mind the frontera is a completely different animal from the Tocumen Airport.  The airport has video surveillance.  Laws change frequently in Panama when it comes to visas and requirements so the best bet is to keep your credit cards and cash available for whatever comes up.  It's why attorneys are always on call & nearby in the frontera.  In my 5 1/2 years living in Panama, tourist visas went from 90 to 60 to 30 and back to 90 days.  Entry requirements also constantly change at the frontera and the attitude of the agents are very "high school hall monitor" so keep smiling and be gracious.  No eye rolling.  "Conejos" abound and often speak English.  Conejos is Spanish for rabbits.  Those are the guys who live in the frontera and are 95% usually related to the guys working behind the windows checking you in and out the country on both Panama & Costa Rica sides.  You'll see them running around helping the bus drivers, taking Passports here to there and are basically self serving "gofers" who are unofficial workers with lots of influence.  They can be nerve racking at first, but are usually well known in the frontera community.

Best of luck.

Hi Elias, i have a spanish passport, same requirements as yours. I got a bit confused, what is the amount of time you are allowed in the country? Have been told is 6 months, can you confirm that??

Thanks

Raquel

visas are good for 180 days, but you can always check with the us embassy in panama to be doubly sure. the hubs and I were only required to have a 1 way ticket purchased and we verified with border personnel that the visas are actually good for 180 days. I think it just depends on who you get at the border window when traveling by land.

Gracias MeanderingMiss,  i will check with the embassy!