The easiest way to get to the island is to fly direct to Volos and then you can either drive around the whole of Pelion or take a water taxi.
FROM VOLOS AIRPORT
A taxi can pick you up from the airport and drive you to “Amaliapolis” also called by locals “Mitzela” (about 25/30min ride), where a water taxi can pick you up and take you directly to Trikeri Island (ard 25 min).
A taxi can pick you up from the airport & drive you to the point across the island called “Alogoporos” beach, about 2 hours drive. From there in a few minutes with a water taxi you will be on the island.
Only for summer period, you can take a bus from the airport to the main bus station of Volos and then get the local bus from Volos to Trikeri (check schedule) that will take you to Trikeri Village. From there you will take a taxi and go to “Alogoporos” beach (10 min drive) where a water taxi will take you to the island in few minutes.
Useful info: Bus from Volos
Tip: Alternatively you can hire a car instead of taxi for the above options. Car hire at Volos Airport
FROM THESSALONIKI AIRPORT
You can rent a car and drive down to Alogoporos via Volos (about 287klm). That takes about 4 hours, costs about 15 Euros in tolls and just over half a tank of petrol. In Alogoporos you can park your car in private parking areas and call the sea taxi to take you to the island. Useful info: Car hire at Thessaloniki Airport
You can get a bus from central bus station (Ktel) to Volos and then change in Volos and take the next one to the village of Trikeri in the mainland (see above). Useful info: Bus from Thessaloniki to Volos or Bus from Volos to Thessaloniki