Anything that you consider fun can be made more fun by doing it with a child. I find that seeing the enjoyment that a child gets from learning and participating is so rewarding.
My favorite activity with my son is fishing because its one of the things that I thoroughly enjoy. I've found certain considerations and methods that work well when teaching my child how to fish.
Choosing equipment for your child is pretty simple. The local walmart will have plenty of child sized fishing poles with their favorite characters on them. These usually come spooled with line and have a practice plug. Obviously you don't want to tie on a hook until you are sure your child is old enough and skilled enough not to cause harm to himself and you. If you really want to add realism you can tie on a plastic worm to your kid's line.
Speaking of equipment, its a very good idea to place a lifejacket on your child while fishing. Sure they won't drown quickly but you certainly wouldn't want to take the risk. There are plenty of hazards so be careful. If you live in an area with venomous snakes you will want to keep an eye out. Rocks can present a trip and fall hazard as well.
Bring sunscreen and bug spray. We usually don't need it though because children's attention spans are so short but you don't want to need it and not have it.
Another thing to consider is location. There are tons of ponds all over the US so finding one should not become a major event. You can try to fish at a park if they allow it. Usually they won't boot out a child with a toy fishing pole. You need to find a location that is generally flat and with still water. I don't take my child to the creek. If he falls in the water will sweep him away. I also stay away from overly weedy areas or areas with logs and stickups that can cause hangups.
One bad piece of advice I hear about taking children fishing commonly is not to bring a pole of your own. This could not be more wrong. Bring a pole otherwise the child will not get to observe and will not feel part of something. Believe it or not, children don't want attention all the time. There is another reason for this. Children want to be rewarded. They may not be able to catch a fish at first but if they see you do it they will realize that work yields rewards.
The teaching is a little more complicated and sure to vary from child to child. All children's starter rod/reels are spincast models so its as easy as showing the child how to press and hold the button then swing and release. I was surprised at how quickly my son picked that up.
You do have to be very patient. They will get hung up a lot. They get tangled, break things, lose things, etc. Its important to understand this. You don't have to leave your rod/reel at home but you certainly must be prepared to lay it down. Show the child what went wrong. They understand more than we suspect.