Huanglong gets mixed reviews and particularly from foreigners. My research determined that for me personally, it was of much less interest. Strike one. And it takes another full day in a bit different direction than Jiuzhaigou, and I would have had to stage logistics for a night in a different location (Chuanzhusi rather than Jiuzhaigou itself). Strike two. Finally, I had been on the road hard traveling for nearly one month and was tired, and knew I would use my entire quota of hiking ability and stamina in one day at Jiuzhaigou. Strike Three. Huanglong got the heave-ho. However, others with more time, stamina, and/or cash might come to a different conclusion and particularly if fall colors have started. Huanglong has a separate entrance fee of RMB 200 for the day, plus cable car fee of RMB 80 (peak season except winter), so it's not a cheap date.

There are plenty of small supermarkets and restaurants in Pengfeng Village which is where the park entrance is located. I stocked up on non-perishable goodies the night before, plus a starter stash of water. There are a LOT of hotels/hostels in Pengfeng. The next closest area that also has a lot of hotels is Zhangzha town a few km away, it also has some big supermarkets for stockup.

Note: There is no xray or WTMD to get into the park, so no security/confiscation issues there. Take a small daypack with your eatables, sunscreen, tissues, etc. Again, there is a cheapie snack area with an open (covered) table area at the Nuorilang Tourist Service Center inside the park; the selection isn't great, but you can augment your stash with something quick and hot, without having to deal with time and expense of the indoor sit-down restaurants.