I am teaching an undergraduate course which consists entirely of exploratory projects on any interesting mathematical topic. I'm looking for projects where a first-year student who has only taken calculus can make discoveries within an hour, and an advanced undergraduate thinking about graduate school can continue even after twenty hours. I prefer projects where students can do short investigations, make conjectures, and then try to prove them (or run computations).
So far I have found the book The Joy of SET, which I think is ideal for such projects. I am also thinking about projects on prime numbers, cellular automata, and lattice paths.
I would like additional resources that I can work from, where the authors have already thought out how to prompt students to explore topics with particular low-hanging fruit in mind, and tons more fruit beyond it.
To be clear, I'm not looking for textbooks for students to read themselves as in Recommendations for inquiry based/aided discovery textbooks, although I got some ideas there (Indra's Pearls). I also got some ideas from Topics for Discovery-based Projects, such as Julia Robinson Mathematics Festivals.