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Ah, shit, that first one sounds familiar. Haven't thought about it in years, but I remember I read a lot of cul-de-sac kids (and Encyclopedia Brown, but that's probably not it) so that might be a starting point. I'm not finding any connection on Google though.
You know what, now that I think about it more, the second one is definitely something that Encyclopedia Brown would have in it. A quick search shows it's one of the stories in Encyclopedia Brown and the Case of the Disgusting Sneakers
But that first one... this is going to eat away at the back of my mind for half a week....
The first one is also an Encyclopedia Brown story. I don't remember which book it's from, but I know that it's definitly from that series, because I remember reading it. Most public libraries have a ton of Encyclopedia Brown books, so it shouldn't be overly hard to find if you have the time and patience to read them all.

1) A young adult novel (or possibly part of a collection of short stories involving the same main characters) that was probably in the Kid Detective genre. The main characters are involved with some sort of townwide scavenger hunt and one of the clues is "a famous dairy". They go to the only well-known dairy in town and find nothing, then figure out that someone sabotaged the clue by switching a couple of letters around - what they're really looking for is a famous diary. This could have even been the other way around, actually.
2) Another detail I remember from a different Kid Detective type book: someone's reported missing and the only thing unusual in their house is a note reading "crabcakes". Someone figures out that, because there was also a van sighted nearby with the ABC logo, "Crabcakes" is actually a coded clue left behind by the victim.
Edited by MikeK