Substituting fresh vegetables/fruits for canned I've made a commitment this year to reduce the amount of waste coming out of my home. This includes cooking and buying in smaller portions so less is composted and paying attention to what kind of packaging my food comes in, so it can be eliminated wherever possible and recycled other times. Part of this is I'd like to drastically reduce the amount of canned products I buy. I know cans can be recycled, but I figure it takes energy to convert that can into something else, so it's still better if I buy less of them. 

In attempt to reduce the amount of failed trials with trying to substitute fresh produce for canned, does anyone have any tips as to where my efforts may not be worth it?

For example, my sister is telling me that if a recipe calls for canned tomatoes, it's usually better to just listen. Another recipe I tried to sub fresh pineapple for canned in a dessert and learned that the acid in fresh pineapple destroys the gelatin proteins so that is why my dessert didn't set. [Edit: It's the enzymes in pineapple, not the acid. I actually knew that and just mis-typed, but thank you, Joe, for the correction!]

If it helps, I live in Vancouver, BC, Canada (to give you an idea of what is grown locally during what season). I have not ventured into canning endeavors yet, but I do have a stand-up deep freeze so I freeze a lot of stuff myself. I do not have a garden, so everything is bought from a store that sells organic, local-whenever-possible groceries. 

Thank you in advance!
Tomatoes are one of the few things I use from cans, mostly because they have more flavor than the ones you usually get at the grocery store, but about 2-3 good-sized, ripe tomatoes substitute just fine for a can.

The canning process includes heating the food being canned, so using fresh fruits and veggies might require more cooking to get the same effect or texture in a dish, and as was noted in the comments, could also cause some other effects like denaturing enzymes. If you're preparing a recipe that doesn't require you to cook the canned veggies, you may want to steam or blanch their fresh counterparts before adding to the recipe.

I find the can also affects the flavor, but not in a good way, so I wouldn't worry about that difference ;-)