The closest airport from Peniche is Lisbon Airport (about 90k). Flights into Porto might be cheaper, but it will take up to 7 hours+ with public transport to get to Peniche. Don´t expect the trip via Porto to be much cheaper by rental car than flying into Lisbon. The highway to is more than €20 per way. With gas prices raining in the most expensive all over Europe the 300k will cost you around €30 even with a small car.
Germanwings (now Eurowings) has direct flights from Köln, Stuttgart, München, Leipzig, Hamburg, Berlin.
Easyjet from Berlin und Basel.
Airberlin via Mallorca from many major cities.
TAP Portuguese airline, usually the cheapest option from all over Europe since luggage is included in the fare.
Swiss all Swiss airports.
Ryan Air from Frankfurt Hahn, Bremen und Hamburg straight into Lisbon!
Skyscanner.net will search for the cheapest flight in a chosen month.