Hey Marg. I'm far from qualified to give advice on what to plant, but what about some lavender? I think their color might look nice with what you already have there (??) Then you could put some low growing annuals, if you do annuals.
Or, what about burning bush (??) The red might go nicely.

Do you have a picture?

I had thought about annuals but they are far enough back from the laneway that you wouldn't really be able to see them. I also have 2 burning bushes. I don't know why they aren't really growing, I've had them 7 years and they aren't much bigger than when I bought them, but I think a shrub with colorful foliage would be great! Never thought of that. I also thought about bulbs around the outside of the clump, perhaps daffodils? What do you think? Marg

Daffodils would be awesome. I'm a big fan. I used to live near a daffodil farm. Fields full of daffodils =:) Daffodils almost as far as you can see =:)

The only thing with them is what they look like later on. Would wormwood or something like that go there to disguise the mess that the daffodils leave when they're done? Wormwood might be too close in color to the junipers?

I hope it's alright if I jump in here. I tend to be more of a lurker than a poster. I think a dark-leafed shrub like 'Diablo' ninebark or the one of the red barberries would be a beautiful contrast to the white of the birches and the blue-green of the junipers. Daffodils would make a nice edging, maybe interplanted with bold coloured daylilies to disguise the foliage later in the season? Or some of the taller zinnias that make a statement from a distance, if you prefer annuals.

I was thinking naturalizing bulbs as well. Sounds like a perfect place for a bright spring woodland type display. Purple, white and yellow color combo would be very striking, I think.
I love birch and I'm planning on getting more in the ground around the garden, too.
This isn't a native suggestion but Brenda (aka: valleyrimgirl)had a photo of her peonies growing amongst juniper and it always stands out in my mind.

Prairie_chick, I had completely forgotten about the ninebark. I already have some but I would probably buy several larger ones to fill in quickly. White birch/mint colored junipers/darkleafed ninebark. What a combo that would be!
Lynn, I could always plant the daffodils first and then plant several taller daylilies in front of them. That way the daylilies would hide the dying foliage of the daffodils.
I don't know how much I would get done this year, my arm is still in a sling, but I can get started. Thanks for the ideas. Marg

Are deer a problem in your area? Last fall the deer here really enjoyed the ninebarks I planted last spring. Pruned them back by half, and they were doing so well.

There are several sites you might look at for info. If you google "native plants of Ontario" there are lots of links. One that looks good is: http://www.ontariowildflowers.com/

Another site that might give you ideas is: http://www.perennials.com/results.html?searchmargin=atwildflower%7CWildflower

Some easy care plants I grow that might work are yarrow, perennial blue flax, various poppies, columbine, catmint, and lavender hyssop. The deer don't seem to bother any of these.

A commercial site is Wildflower Farm. They don't ship and may not be near you (they are near Orillia) to purchase but you might get some ideas.

Here is a link that might be useful: Wildflower Farm

I forgot to mention dogwoods. I have planted red and yellow twig dogwoods with a birch and if that birch ever gets its white bark, I expect it to look spectacular in the winter. There are spruce behind them as a background. The birch has met with several husband and dog related mishaps and has only managed three feet of growth in the last four years.