Think about things you find soothing/calming, or that can make you feel relaxed or content or absorbed in what you're doing. If that feels a bit much to think about, maybe ask yourself about the places and activities you gravitate towards. Are they in places that tend to be quiet or busy? Do you have definite things planned to do or do you prefer to make it up as you go along? Are you happy getting around by foot or do you prefer to have public transport options? Holidays and breaks are simply a chance to do whatever feels most natural and restorative and comforting to you - it's a very individual thing.
Once you've thought about that, you can use it as a guide. You like outdoors, countryside, peace? Maybe go to the coast, or to somewhere with hills and lakes or forests. With that kind of place, you might want to check on the best ways to get around, thinking about whether you would want to go out to visit a particular place or two or whether you could just, say, pitch up in a village by the sea and have a nice time reading in coffee shops and wandering along the sea front and things.
If you prefer being closer to civilisation, maybe look into places with interesting museums, or galleries, or libraries. Any of those can be great for wandering around, just seeing and thinking about new things, in a very relaxed low-pressure way.
Think about how much of the time you would want to spend in the place you're staying (in which case you might want to consider whether it's somewhere quiet, what the view is like, whether there are cooking facilities, etc.) and how much you're likely to spend out and about (in which case you'd be looking more at transport options, local food places, sights to visit, whatever).
I hope that doesn't all sound like too much organising/a lot to think about! You really can do it in as high- or low-octane a way as works for you. Things like getting an open return train ticket might help take the pressure off, because that means you have a fixed train there but can get one back whenever you like, so less need to plan that.
If you have any other questions then say! I've done a lot of going places, I can do my best to help.