Argostoli is the capital on the mid-west flank of the island and is not really a destination itself, that moniker falling to the contiguous beach scene running from Lourdata south eastward to Skala. While development might have caught up here by now, it should still be a pleasant beach scene and focus for a relaxing stay. The Melissani Cave on the east coast is a must attraction and while it can be visited on a day excursion from anywhere on Kefallonia the two villages or Agia Efthymia and Poros are low-key ‘resorts’ pulling in a regular crowd of travellers and may warrant a look-in. The port of Pesada (just west of Lourdata) is the home of the local ferry to Zakynthos (Zante).
You seem to have your heart set on Zakynthos but I have to say it’s not a good use of time for a sort-of overrated payoff. Your time is much better spent (in my opinion) seeing another Greek island in the cyclades (maybe taking a ferry to Naxos or Paros sitting outside on the deck drinking a bottle of wine) rather than taking a bus and connecting flights. Zakynthos and Santorini/Mykonos are on opposite sides of the country and the only way from one to the other is by flying (or some combination of bus and ferry). So, my recommendation is to consider spending those Zakynthos days in Naxos, Paros, Milos, or some other Cycladic island.
I loved you website, congratulations. Me, my wife and a couple of friends are traveling to Greece in march/2018 (I know it’s not the best time of the year to visit Greece, but it’s the time we have available). We are in our late 30’s and are looking forward to some beautiful views, chill out time and good food. We’re not after huge parties but some local music wouldn’t be bad. We have 13 days in our hands, so by reading your texts I’m considering spending some 2 days in Athens, 3 days in Mykonos, 4 days in Santorini, 4 days in Chania.
The almighty church has kept nightlife in check. If you must see and be seen, head to quietly glamorous Astivi or Stoa bar, on miniature Agia Lesvias square, in Hora. Beach life is generally languid and low-key; Psili Ammos and Livadi Geranou are our favourite hideouts. Dinner reservations are essential at Benetos, for Med-Asian fusion on an organic farm, and Lambi for grilled fish on a purple pebble beach.