I might avoid doing the tiny furniture, honestly. It's a really difficult subject because it's so hard to tell with the fae what they like sometimes. It is my personal belief that fae can come in any shape and size but I also know from past experiences that they really don't like others assuming they're tiny. I was contemplating using the furniture too, because I thought it was really cute but then I wondered if it might keep them away rather than invite them. It's up to you, though! If you have a relationship with them already and you feel it's okay, then go for it!
As for the garden itself and other things you can use I can help with that. (: I had one a bit ago before the flowers actually died. Dx Some things that I'd suggest using are shiny rocks and stones like polished gem stones, plant life of course, little statues of fairies (which again, it's so hard to tell because they'll be the tiny ones with the wings but I suppose you can always remove it if you want! I've never had problems with keeping my little statues up), adding feathers is good too. For symbols, you could incorporate the Sidhe Sigil (I think it has another name but my memory is terrible) which is right
here. It's believed to be used to open the portals between the Fae world and ours depending on the way you draw it. I'd also add in some symbols or representations of the elements as well. These are all things I've done myself. ^_^
Another thing I have done that was a lot of fun was I took little tiny glass bottles and filled them with water then added sparkles to it. So it made a really pretty sparkly liquid that when you tilt it this way and that it shines in the light. They have little corks on the top to keep them closed too.
I love the idea of this being in the wishing well, though! Thinking about hanging things you could easily take pieces of colored glass and make a hanging mobile of some kind that will glitter in the light. That might also appease your entity you are friends with, due to being associated with the sun.
That's all I can think of off the top of my head! (: I wish you lots of luck and would love to see a picture when you're finished if you don't mind sharing it!