Part of an Unbalanced Diet

We're on our way to Puyallup, an old home for me. I had though we were going to the fairgrounds, but we're actually heading to Pioneer Park, which is right next to the library. We run into some issues involving one-way streets, but we eventually find our way to Pioneer Park. I notice other team vans parked there, so I know we're not doing too bad.

Team Ultraviolet splits up and starts combing the park. We come together; we can't find the puzzle. Other teams are working on something, so we know it must be here. I try asking a team for help, but they rudely refuse. Very well. More searching happens, and eventually one of our taller team members notices the puzzle hidden in some trees. We have another story pyramid and a bag of M&Ms.

The bag of M&Ms looks normal, except a new nutrition label that has been taped on. It includes the message "All daily nutrition labels are part of an unhealthy diet." What do you want to bet this is a clue?

We go ahead and open the bag to look at the M&Ms. They are all pink and green, and have some sort of encrypted text on them. The pink M&Ms say "HCZXTZ JK" and the greens say "YLQRRZ RRH CGIO" This isn't another simple shift cipher; it's unlikely something starts with a double letter. One team member decides to go across the street to buy a real bag of M&Ms for kicks, while the rest of us get cracking on the daily nutrition info.

I notice that there's some strange things about the nutrition information. Some nutrients don't have DIV percentages, and all of the ones that do are bold. Some nutrients don't make sense for candy; for example, Fiber is listed. All percents are low, I don't think any were higher than 5. We decide to use the percents as indexes into their nutrient and come up with "Vigenere." I think this looks vaguely familiar, but we wonder if it's an anagram.

Our missing member walks back, distributes candy for us to enjoy, and looks at what we've written.

"Vigenere? Like a vigenere cipher?"
"Oh, I know what that is!" I cry. I'm a little fuzzy on the details, but I start writing the huge alphabet block for a vigenere cipher. We decide to go up to the Starbucks on South Hill to look up how to properly decode the cipher, while another member calls his brother and asks for more info. I hear a word or two and remember how to decode the ciphers by hand.

Vigenere ciphers require a key, and I assume it's the color of the M&Ms. I have the first one decoded before we pass the fairgrounds; "SUMNER WA"

"Guys, we're heading to Sumner next. Give me a minute and I'll tell you where."

We make it up South Hill and park as I finish the second M&M; "SUMNER AND PARK" We need to go to the corner of Sumner and Park in Sumner. Sumner is next-door to Puyallup, so it'll be a short drive to the next puzzle. We call up Game Control to get the OK and get on the freeway.