Well, I know that fish like to hide among the plants, or sit with their faces in the bushes, so I was thinking about adding a hidey place. But I think a lot of them are quite ugly. What do you guys use - I don't really want a plant pot in there.

just make sure that the water does not get trapped in the hidey place then i becomes stagnet and grows very bad things.
i have a jar that they can swim through or sit in

well, you can put a lot of plants around a corner with an open space in the middle, you can also stack some larger rocks.

I've found, though, that if the plants dominate an area too much, and get too thick, the fish stop hiding in them.

I find they like to hide their eyes. Because goldfish cant close their eyes its nice if there is something they can use a kind of mask. Esp. for people who use lights for long lengths of time. I have a small piece of bog wood that is low but they can go behind for shade/ eye rest.

I think someone said you can hot-glue rocks into a kind of cave/tunnel thing...

I tried it, but it fell apart. My tropicals have a small glass cup now.

I used several different sized terracotta pots some for the fry, a largish size one for the big guys and a very large one everyone can fit into. I also use large decorative stones (rainbow stones) for them to hide behind as well as driftwood and very large fake plants.

I do not see the need to "hide". My fish enjoy company and will swim to the glass anytime they see a person. They do not hide. Even the tiny 1 month old fry are quickly beginning to respond to human presence.

The plants that are in the tank are used to "anchor" a fish during sleep. Many do seem to enjoy getting "stuck" in the soft leaves of a plant when they sleep. In fact, if for some reason I leave the lights on too long, they all go climb into the plants like a bed and go to sleep anyway.

I have bare bottomed tanks. I glue stones to the bottom of soft plastic plants and place those in the corners for "beds". They like them.

Tanks that are "bare" are also fine - as long as there is a place where there is little or no current - where a fish can hover without having to move a fin to stay stable. They want to sleep - not swim against a current. A plant can block the current. Or the tank can be designed to have these "sweet spots" without additions.

I do NOT like hidey holes - what a fish fits one month will be too little the next - but the fish will still try to jam themselves into it. I have read far too many horror stories about fish jammed into spaces - getting killed or seriously injured by doing so. The fish continue to grow all their lives. They do not realize just how big they are, though. The hidey hole will not grow with them. I try to prevent having ANY closed space that does not have "give" in it within a tank. Even the intakes are designed to "float" to a degree, incase a fish decides to try to jam themselves between it and the tank's side!

Daryl's advice is great.

When I've had fish that liked to hide, it was generally after an injury when they wanted to hide from me, or because they had an aggressive tank mate who was bothering them. I like to make sure the smaller fish have somewhere safe to hide if I'm introducing new fish to the tank for a few days, but generally don't worry about it too much in the long run.

Mine do love to sleep near their plants, one nose down in the java fern to keep herself immobile, and a few perched in a sword plant. And in rooms here the cats are, I generally have some plants they can hide behind to be out of sight, and that seems to be just as good as having a little cave or something for them, even for nervous fish, and I don't have to worry about them getting stuck. The "if I can't see you, you can't see me" rule seems to be what they want.

I have a little cluster of fake bamboo and a little fake lily loosely bunched together. My fish like to hang out behind them. I don't even have a light on my aquarium but everyone needs to chill once in a while, yeah?

So they go behind their plants. *shrugs*