When I was growing up I always thought that ADHD meds were for the parents and the teachers. You know, to calm the child down so that the adults didn't go crazy :-) While there is a bit of truth to that, the meds are for the child. ADHD is very different in each person, but very simplistically put, it means her brain has trouble adjusting to stimuli or settling down and focusing. The medicine helps her brain to slow down and focus. My point is, you are not doing her any favors by taking her off the medicine during the summer. If she truly has ADHD, this doesn't just affect her at school. This affects her entire life. She really might function better if she's taking the meds.
People with ADHD very often have something else going on in addition to ADHD. My 5 year old has been diagnosed with Social Pragmatic Communicative Disorder (which a few years ago would have been Aspergers, so sometimes I just say he has Aspergers), and his psychologist said he could very likely also have ADHD. It is a great idea to take her to a clinic or other specialist to find out more. Our son's pediatrician has referred him to a Behavioral Pediatrician to get a more clear diagnosis. I am very grateful for this, as I am really having trouble with his behavior. I just want to have a better understanding of what makes him tick!
Our son recently began Social Skills. He works with a therapist once a week, and they have goals for him and a series of activities related to each goal, as well as a way of measuring his progress. I can't remember his goals off hand, but they are something like participating in a back and forth conversation, even if it's about a topic he's not as interested in, learning how to be a good sport even when he loses, inviting another child to participate, being flexible when plans change. I think last week they were going to play Candy Land and set up the whole game and were ready to draw a card and his therapist said, "I changed my mind. Let's draw a picture instead." This was designed to see how he would react to a very sudden change.
It sounds like your daughter would benefit from Social Skills or some other type of social therapy. Some of the things you described are probably simply her personality. She may never love surprises, and it's not fair to try and make her. So it might help for you to notices that things that really upset her and try to respect who she is as a person. (My husband knows better than to suggest we go to a concert together since I hate loud music. He loves it, so I encourage him to take a friend.) Still, it is something you want her to be able to tolerate from time to time. People are going to surprise her occasionally, so it would be great if she could appreciate their intent without having a meltdown. (I have learned to tolerate the occasional dance club and concert because my friends like that. They have also learned that I actually prefer a coffee house and board games.)
I agree that ADHD, no meds and 30 backpacks to choose from is a recipe for disaster. That is too much stimulation for must kids, much less a child with ADHD. It's not just you (or her), though. My 8 year old son has trouble deciding, too. For the most part, I just do the shopping for him and say, "Hey, look at this cool lunchbox I bought you." He, however, does not hate surprises, so that makes it easier on me. Sometimes he won't like what I bought him, and that's annoying to me. He doesn't want to go shopping, but he wants me to somehow know which shirts he would like. Ug!
Maybe next time you could have her look at something else, grab 3 backpack you think she would like and bring them to her. "Ok, we have Frozen, My Little Pony and Hello Kitty. Which one do you want?" Or, you could (not take her to the store at all) take a picture of each of them with your phone, show her the pictures at home by the backpack the next time you're out.
I bet your daughter is a wonderful girl. Seeing a specialist is just going to help you understand her a little bit better. That will give you some tools to help her adjust, as well.
By the way, my 5 year old is also very inconsistent. There are days nothing upsets him, and then there are things that just out of the blue cause a meltdown. It was hard for his PreK teachers - if they could predict what would set him off, things would be so much easier.
Goodness gracious, why can't these kids just be consistent. It sure would make parenting much, much easier :-)