Destinations without cars: Catalina, California
Now here is something that is out of my wheelhouse for sure seeing as how I focus almost entirely on very affordable destinations that backpackers and people with almost any level of money can visit. That is not what Santa Catalina Island is by any means.
This island, which is less than 50 miles from one of my least favorite places in the world, Los Angeles, is definitely not budget friendly. It is however, one of the only places I have ever been in the world that doesn't allow personal cars and only a very limited amount of other vehicles that are, as far as I could tell, there only for delivery of large amounts of goods to the few establishments on the island, and of course the Casino.

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I was visiting someone who unfortunately lived in LA and we had to do something and I am quite happy that she suggested that we go out to Catalina.
This is an island of extreme opulence and the amount of wealth that a person would need to have in order to live here or have a second home here is just something that almost all of us would just have a heart attack if we were to see what that cost might be.
On the other side of the equation though, it is actually possibly to day-trip here and avoid most of the costs. The port that you will likely arrive at, which is pictured above, is walkable by foot and while you get your bearings and have a coffee that is going to cost 4 times what a coffee should cost, you can contemplate very carefully how you are going to spend your day exploring this very small island.

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You are going to feel like a goober and yes, you are just another typical tourist by doing this, but there simply isn't any other way to do it. The golf-cart rentals that come with a map to get you to the various viewpoints is the way to go but this is still quite expensive at normally $50 an hour. That sounds ridiculous to me but my LA friend had become so accustomed to paying way too much for things that this is just what things cost in her world. She had a good job and probably is up to her neck in debt - I'm not sure about that as I haven't spoken to her in many years.
I would advise staying away from the Casino although you can get a very good view of it from any of the many viewpoints that overlook the bay.

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I've never set foot inside of this place, nor a vast majority of the other buildings on the island, and I was only there for about 3 hours in total. Was it a good time? Yes, of course it was and the peace and quiet compared to the never-ending noise and chaos of Los Angeles where we just were hours prior is a big part of the reason why I felt this way.
There were a number of places where we could park the cart and go for a swim and to me this was one of the best parts of being on this island.

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As anyone that has ever attempted to go for a swim off the coast of California can likely attest, there aren't many good swimming places anywhere on the coast of that part of the country. Due to tidal influences that keeps the water rather cold, as well as a mostly rocky and rough waves covered coast for a thousand miles up and down, you basically don't see anyone swimming in the seat in Cali, but here on Catalina, for reasons I can only speculate about, the water is calm and very peaceful. There is also reasonably good snorkeling to be done because there is a strict environmental procedure in place here because, well because it is California so everything is illegal. What can I say, sometimes that strict way of life does some good things.
The main thing to enjoy about Catalina is that it truly is a respite from the craziness that is all the rest of the Los Angeles area. You truly feel like you are in another world when you are here and to me that makes it well worth the $50 or so that the round-trip ticked on Catalina Ferries costs.
It truly is amazing how wonderful a bit of peace and quiet can be after your ears are being constantly assaulted at all times when you are in the city.
Overall I would say that if you can avoid Los Angeles, do so. There are other places that are a lot better than LA and the things you might thing are going to be wonderful are actually pretty crap. Catalina is not crap. It's actually very lovely, extremely well-maintained, and accessible to everyone, well everyone with $50 that is.
If you want to be truly frugal pack yourself some sandwiches and eat them in your golf cart with an amazing view rather than paying $100 for a soup and half salad at the ripoff restaurants that exist in the one town on the island.
I think the day is going to end up costing you more than $200 per person no matter how you try to save money here but if you are stuck in this part of the world I think it will do your mental health a whole lot of good to get away to what little nature and peace exists in Southern California.
it is only one of about a half dozen inhabited but yet car-free places I have ever been and it is the one that kind of stands out the most.