If you like the sea, even if you don't dip your toes in it in March, I agree with Jill and walk along the coast on your first day out of Matosinhos. Her highlights are also spot on, but I will throw another idea, or two, out there. So here goes.
Depending on thetime you arrive in Porto, take that day to explore and walk your first 10km to Matosinhos along the Douro river by crossing the Eiffel bridge by the cathedral, where you can get your first stamp, and credencial if you haven't ordered it feom your national pilgrim association or ordered it from the Forum shop here.
You will then pass by the port maker warehouses, walk along the river giving you stunning views of the city on the other side. After passing another bridge, you will see a small dock where a shittle boat can take you back across. From tnere keep walking, this time with the river/then ocean to your left. You will arrive in the suburb of Matosinhos where you can take the metro back to your hostel/pensio/hotel, visit the old town a bit more or head to bed. The next morning, leave your backpack at the hotel/hostel/pension and get on the metro to Matosinhos again, and then start walkingalomg the coast for a full day to Vila do Conde. In VdoC you can take the metro back to Porto for the night and the next day head out to Barcelos as suggested by Albertinho.
Second thought... instead of following the yellow arrows from Pontevedra on the traditional Central route, opt for the Variante Espiritual. You will find flyers about it in just every albergue. It allows you to visit the beautiful Poio monastery, the quintessencial fishing village of Combarro and then a hike up the mountain (or taxi, 10€ or so, telphone numbers to be found on posts as you come into town, or ask any reataurant to call one for you) to the Armenteira monastery. There you can eat at the excellent A Fonde restaurant and slend the night at the albergue. In the morning walk what has to be one of the most beautiful stretches of any caminos, the Camino de Piedra & Agua, a path along a small river, or large stream, flanked by a series of mill ruins. It's out of a movie. Simply beautiful. This route will also have you take a small boat from Vilanova do Roussa into what is basically Padron where you get back on the traditional Portuguese Camino.
The boat in Vilanova leave at different times every day, depending on which boat and operator it is, and depending on the tide. When at the Armenteira albergue, look on the posters on the wall, or ask the hospitalera, a lovely woman, and call the boats to let them know you would like to get on one either the next day, or the one after, depending on your schedule and their departure times on both days. If you are short on time, you could take a taxi from the pension/restaurant Os Castanos in the town at the end of the Ruta de Piedra & Agua to Vilanova. That will cost you some 20€.
http://www.gronze.com/documents/pdf/variante-espiritual.pdf
In Pontevedra, Jill suggested avoiding the large albergue at the entrance of town. I agree and suggest you stay the Santuario of the appariciomes on calle Sor Lucia, which offers a few rooms for a good price and is very well located. The Pontevedra tourism office can call and get you a room. It's not in the
Brierly guide but in a German one, and my notes are currently put away due to home renos. Maybe someone else can find the phone number for you, or email if they have email.