Panama Visit advice (Panama City)

Hi Guys,
My girlfriend & I are going to visiting Panama for 3 months and am wondering where the best places to visit are.
The plan is to make our way up to Costa Rica over a month or so and upon finding a spot we like, to then stay there for 2 months, ideally in a cheap enough long term rental.
The type of places we would be looking at are beach towns that are not to big but have enough amenities & life to be kept interesting i.e. supermarket/local market, a few restaurants & bars etc.
We just want to relax & take things slow without the hussel & bustle of a city, buying fresh food daily to make lunch/dinner and be able to have a small choice or bar & restaurants to go to when not bothering to cook.

From what I have researched so far Coronado would be too big but possibly Gorgona beach might be a possibility.

Any suggestions, advice or information on areas, costs, rental rates etc would be very helpful, from what I can see on-line vacation rentals do not seem as cheap as one might have thought comparing to Costa Rica, Ecuador etc, ideally looking to pay circa $500-£650 a month.

Thanking you in-advance for any help offered.
Ben