Congrats on the alternative destination :) Alonissos is a very beautiful island. It is the most remote of the Northern Sporades Islands, making it the perfect island to unwind! You can spend your time exploring rugged landscapes rich in history, and ancient Byzantine ruins. You will surely enjoy leisure walks along pine forests, olive groves, and orchards. A magical place indeed...
Alonissos, is one of the largest marine reserves, in the Mediterranean Aegean Sea, and the home of the National Marine Park, a refuge for rare seabirds, dolphins, and Mediterranean Monk Seals. I have to tell you that... we actually saw (!!!) two Monk Seals in their natural inhabitant on our visit! Some friends had a boat and gave us a tour around the island. It was amazing to witness such a creature up close!
Some of the best beaches in Alonissos are:
Agios Dimitrios Beach (one of the best on the island according to the locals)
Chrisi Milia Beach (it means Golden Apple Tree, near Patitiri)
Glyfa Beach (more secluded)
Megalos Mourtias Beach (near Patitiri, perfect when the northern winds blow during the summer)
Kokkinokastro Beach (it means “red castle”, it has colorful sand)
Votsi Beach (a beautiful little bay for the romantic ones... )
Skiathos island is near Alonissos and can be easily combined, you could take a day trip by boat to get there. Another good idea would be to visit Mt. Pelion as well! For your accommodation in Alonissos, you might wanna check some reviews first. I stayed at Kamelia Villas, my father booked it through Hostelbay. It was a bit expensive (100 euro per night). You can find a good place to stay around 40 euros per night as well.
You'll love this island. The fifty shades of green and blue all in one paradise <3