We are a couple in ou 30's, have booked quite a bit of our trip thanks for all of the help so far. There are a few places that we haven't confirmed so I will include the itinerary so far and would love feed back on the rest of the stay.
Nights Fly in on the 4th and drive that day to
4th & 5 th Villasimius
6th & 7th Torre Di Bari
8th through to the 12th with a kayak trip in the middle Lotzorai
13th & 14th Capo Coda
15th 16th 17th nothing booked but we thought La Maddalena
18th Alghero nothing booked
19th Oristano "
20th Cagliari " and fly home from here the next day

We love the sea, really enjoying swimming, kayaking, some tramping and prefer very casual as close to the beach as possible. Food lovers and keen to try good local restaurants.
We have a rental car . i am concerned that maybe we need two nights in Alghero so maybe one less at La Maddalena.
We would rather have a longer drive and stay put for a couple of days in the same place.
Your iput and suggestions from experience would be really appreciated.

If you drive away from the port town of La Maddalena there is more to see than at first glance. If you cross the bridge to the next island north where the Garabaldi estate is located, there are so interesting places to visit. That island has a number of abandoned Italian military fortifications that you can "tramp" up to with rock stop look outs that afford a 360 degree view of the Archipeligo system. Driving along the western shore affords good views of Corsica.
There are many many accommodation and restaurant choices. Hotels Villa Del Parco and Miralonga are close to the water yet slightly west out of town.
Both Alghero and La Maddalena offer many water sports opportunities. The road from Olbia to Alghero is nice and you can visit a former Cistercian Monastery with snack bar just west of Sassari.
While a bit pricey the Hotel Catalunya in Alghero provides good views of the old town and the park if your room is on the upper levels, west side with a balcony.
Two days is adequate for Alghero and a minimum of two for La Maddalena depending on what and where your interests are. I think you will find that Alghero has more of a medieval & Spanish flavor while La Maddalena is more Italian with varied & diverse offerings.
As you likely know Sardinia can be very hot and the mid day sun brutal, although brisk summer winds often help one to feel more comfortable.
Try to avoid the ferry from Palau to La Maddalena during commuting times. Usually late mornings and during reposo hours (12-4 are best. Good luck

Thanks Parmenter I appreciate your input. Are there any coves or beaches that are an absolute don't miss before we get to Alghero?

Yes, indeed the coves and beaches of La Maddalena, especially the 'Parco Nazionale dell' Arcipelago di La Maddalena, offers wonderful beaches with colors that seem not quite real. Of course Costa Smeralda has its' own exquisite beaches and also a rather robust nightlife. (El Peyote).

Also, Don't forget your own 'robust' sun block. Ralph

I live near Villasimius and I can tell you that Villasimius and Castiadas beaches are worth a visit !!!