Day trip to Hydra or other suggestions for an island out of Athens or Sunion?

It's been over 20 years since we stayed on Hydra so my memory may be a bit sketchy. Many of the 'streets' on the island are more like staircases. When we hiked up the hill we started in town walking up these streets (stairs) before it became more of a hiking path. It wasn't horrible but it was just a bit strenuous although if you're ok with stairs at home it shouldn't be too bad, just take it slow especially if it's a hot day. I'm sure there are more paths and maybe someone will have more information. We hiked up to one of the monasteries up on the hill and it was worth the time and effort.

For a little less hilly and rocky island close to Athens you can check out Poros. It's a shorter ferry ride than Hydra so more time on the island and it's bigger than Hydra. There are lots of places to check out and things to do on a day trip here. We were only on the island for a little more than an hour so didn't get to explore much away from the harbor. And again I'm sure others will have more information.

There is only one means of transportation on Hydra: donkeys. All motorized vehicles are prohibited except for a small garbage truck.

The village of Hydra is on a hill, so there are slopes everywhere and sometimes stairs. Hence the presence of donkeys.
These donkeys can take you for a ride for a few euros.

"go up the hill to see more of the true Greek Island”" sounds like an AI-generated sentence. You could see the real Hydra from the docks. No matter which island you go to, if you stay near the port, you'll inevitably see the more touristy side. In Hydra, you'll have no choice but to stay near the port/village since outside the port, there's nowhere to stay.

Aegina is a closer island with fewer slopes. It's also a tourist destination for visitors staying only a day or two.

https://www.aegina.com.gr/?lang=en

There are no ferries at Cape Sounion, you have to go to the port of Lavrio, which is about ten kilometers away.
From the port of Lavrio, there are ferries to Kea or Kythnos (1 hour and 1.5 hours, respectively). Kythnos is the most Cycladic-style island.

https://www.kythnos.gr/en/

Anyway, you can't really visit an island by staying only a day, especially if you're looking for a less touristy experience.
Day trips to islands are, by definition, meant to go to touristy spots. So, in my opinion, it's more a matter of time than the choice of an island.

Thank you both.. great information.

Your best options for an island for a day trip would be one of the Saronic Islands as they are the closest to the mainland.

Most would recommend Aegina as it's the largest with more to do. Aegina Town is nice but can be very crowded on weekends. Nice back streets, long waterfront and lots of options for services.

Poros is another lovely island with a very long waterfront but the best part of the island is uphill away from the harbor with classic Cyclades character. White washed houses with colorful trim, narrow lanes and nice views. Alas it's uphill and can be steep in places but that's why it's less crowded as most visitors stay along the waterfront.

Hydra is very popular as a day trip and somewhat similar to Poros with wonderful waterfront but if you want more Greek character it's uphill away from the crowds. It's steep in places but more laid back. There is a nice walk from the waterfront along the coast to a small fishing village, less crowded, and easy walking.

Also Agistri, close to Aegina, more family friendly, easy walking and a nice vibe.

Spetses was just OK for me but many like it.

I've been to Aegina, Poros, Aegina and Spetses and can recommend either one but if I had to pick it would be Poros.