Surprise Trip to the Cirucs??

How old is she? That would make a huge difference!

Does she have a particular interest in circus entertainment, animals, etc.?

Just update your question adding "ETA - Edited To Add" and put in additional info.

ETA: If she is 7, buy Dr. Suess' "If I Ran The Circus". A scavenger hunt is a cute idea - animals from the dollar store or toy store are one idea, but there are other circus acts besides those involving animals - see what you can find at the scrapbook section of the craft store, with stickers and "embellishments" - you can probably buy a pack of "circus animals" and maybe some with acrobats or trapeze artists. Stash them around the house. You can include some money with each one, say a bunch of $1 bills, which she can use to buy the fun but expensive stuff they sell there, like light sticks and so on. If she doesn't have a wallet or little purse, buy one of those too. Maybe her ticket (or a photo of the tickets) could be in the purse or at the last hiding place.

Are you saying you want to surprise her before going to the circus?

If not, I suggest that telling her you will have a surprise for her soon will build her enthusiasm. She will ask questions. Make your answers fun without giving too much information. In my family, it's understood that the person may lie. Part of the fun was listening carefully, trying to know if the answer was true.

Kids that age like scavenger hunts. We do them at Easter and at birthday parties sometimes. Spread the clues out and yes, you could put like a plastic tiger toy as one of the clues, etc. I haven't taken our kids to the circus so I'm not up on what kinds of things they show, but for a 7 year old, they could keep the toys you hide as part of their birthday enjoyment. That's what I've done.
Give them a little bag to collect them in.
If you Google Circus party ideas or something like that on Pinterest or internet. they'd give you ideas of clues you could use with scavenger hunt (popcorn, balloon, clown, hula hoop, etc.) Just go to Dollar Store and pick up the supplies. You don't have to go overboard. Kids just like the scavenger part - but little trinkets along way gives them things to play with for a bit.

An easier alterantive would be a book about going to the circus.

Or a card, cut out their faces and stick them on a picture of a circus scene ...