I think everyone's preferences are going to vary. Mostly, when we've had the DDP, I don't go crazy worrying about maximizing out our dining credits, but we try not to use credits for very inexpensive items.
The last time we used the DDP, we ended up with a few 'leftover' credits. One way I like to use those last snack credits is for something I can bring home. One good bring home options are the mint tins (sold at gift shops). If your last day will be at DS, then at Amorette's you can use a snack credit to buy a Mickey-shaped brownie. The brownies are usually only displayed by the register or not on display. They don't look like much, but they are very tasty. If MK is your last day, then the Confectionary in MK has some great options.
Another useful option is the prepackaged snacks offered in some of the gift shops, like the prepackaged gummies and cookies. Just make sure you pay attention to which ones are included, and which are excluded. The prepackaged items might not be super exciting, but if you are planning to stay (very) late in the parks, they are sometimes one of the only options for food at all. They are also a good take home option. Look for them in the big gift shops, like the Creations shop in Epcot.
To answer your last question, when it comes to food, HS is my least favorite park. About the only food we like is the Docking Bay QS. A Mickey Bar is always a good use of a snack credit, and you can get those in HS and at most resorts.
Another good way to find top snack credits is to look over the Mobile Order menus. Often the 'seasonal' treats will be featured at the top of each menu, and we have found some of those to be among the best snacks. They vary from month to month though, so I can't suggest a specific one right now.
