I love to use my antiques outside in my garden. If they get ruined that is OK. At least I use them and enjoy them. Antiques give a look of history, and foreverness. Try to use antiques in different ways, such as the antique enamelware roasting pans used outside as a pedestals for plants. Signs of all types are great outside. Birdbaths get so dirty and slimy with water in them, especially in Florida so I like to place pots of flowers in them. They make great pedestals. I also place the metal or plastic advertising letters that came from sides of buildings. They are huge in indoor decorating and now make a great look outside too.
Make sure you use some antiques, collectibles, and indoor items for decorating outdoors. Paintings are great to use outdoors but I try to use under an overhang or canape(they still get wet just not drenched in downpours). I always varnish them with an outdoor varnish to make them last longer. Sometime I display paintings outside to give them an aged look than bring them in my house. In this case I don’t add extra varnish, I want them to age and look old.
When placing items in garden don’t always place out in open. Place some items behind plants, rocks,etc, so they are peeking out from behind. This give garden and yard a more natural look. Give the garden a look like it and your display items have been there for many years. Everything doesn’t have to be right out in front in the open, that give is a fake look, a “placed” look, not a natural look. You want a look that makes people think the display items have been there for years and the plants grew around them.