On the topic of finding 100 species in a day, last weekend I went with a friend out to take some photographs, mainly of wildlife.
At the end of the day, we sat in the car and were concluding that we hadn't really seen much. A pleasant enough day, but minimal wildlife.
Then I said, "Ok, let's count how many bird species we saw today".
The answer was 32.
Throw in at least a dozen tree species (probably more like 2 dozen), a pile of herbaceous plants and grasses (even in pre-spring conditions there are lots to see), mosses, liverworts, ferns, sedges and rushes, some fungi, some slugs and snails, and a few flying insects, and there's your 100 species without really trying.
The thing I like about this challenge is that you don't even have to completely identify the species. For example, if you are no good at identifying mosses (like me) you can just say "the green spiky moss on the woodland floor, the slimy moss on those rocks, and the prickly moss on the tree branch". So long as you are certain they are different species it is fine. It isn't a test, just a fun thing to do.