My first advice is not count on any specific character because even with the app, DL is infamous for changing up the schedule & having characters unavailable at a moment’s notice. That being said some of the great bang for your buck characters are the Royal Hall (either in the first or last hour of operation), Star Wars Launch Bay Characters, the Frozen characters in the Animation Building (before 12pm-1pm as they will have Olaf & Kristoff in the morning meeting just after the 2 sisters),& Winnie the Pooh & friends in Critter Country (one line for 3 characters).
In general your best bet to meeting characters is keeping an eye out for characters coming out from . We used the app to plan a lot of our character day & while it was helpful in knowing the trends of times, the characters were super unreliable about being where they were said to have been per the app. Our best meets were the ones that we happened upon unexpectedly.
There are a lot of Disneyland characters that rather than traditionally setup a spot & meet will roam & that will mean they are looking to interact with you more than pose for photos & those can be just as fun if different than a traditional photo meet. They do easily sign autographs (while walking!). Some places to keep an eye out for un-announced characters are at the Mickey floral garden right when entering, along the Matterhorn way from Pixie Hollow until the Teacups/Edelweiss snacks. Sometimes we have seen Peter Pan wandering around Fantasyland early in the day near the teacups/Dumbo without anyone noticing him & that is a great way to find him & interact with him.
If your kids love Mickey & friends, ToonTown opening would be something fun for them. Since ToonTown opens 1 hr after the rest of the park, you can head to the gates 5-10 min before it does & 1-3 characters will come to the gates to open the land. Sometimes coming into the waiting crowd & wandering/interacting with the kids assembled & waiting. They can also take a child or two by the hand & lead them to their first official meeting place & you can get up to 3 of them done in a short space of time. We love going & heading to Mickey’s house very first, then Minnie’s house, then looking for any others that are meeting around ToonTown. Sometimes since people are also focused on getting the rides & there aren’t usually many people lined up for ToonTown opening you can find a character roaming around looking for people to come interact with & meet. It isn’t always the most efficient to make it back to ToonTown after just 1 hour in the park, but this sure is a fun time for us when we do make it up there.
It’s a really fun time meeting characters & I would definitely agree that on busier days (ie longer operating hours) there are more characters out & about & the ones that are there tend to be there longer than normal (ie doing a few sets more in the afternoon or one set earlier). A word on the earliest sets announced: for some reason, whenever we try to make it to a character’s first set time for meets of the day, it almost always involves a ton of delay & often cancellation. Sometimes though, the CMs will say “Oh, that character isn’t coming until X time” and then we stick around for a few minutes to plan where we want to go next & poof the character appears. I believe that a lot of that mis-information is to prevent lines from forming before the CMs want there to be a line. So just know that if you rely purely on the CM information even first-hand in the park from a “Ask me About Characters” designated CM, you may get burned, so really just going with the flow of it & be excited for who you do find!