I'm designing a cryptography-breaking assignment for a college-level introductory security course, and I'm looking for a hash function which is reasonably easy (but not too easy) to generate collisions for. The assignment is that the students are given a hash table implementation which uses such a function, and their task is to design a series of inputs which will tank the hash table's performance. I was originally thinking of modelling it off of PHP's associative array attack from a few years ago, but it's actually too simple (for integer keys, the index used is literally just the integer modded by the table size). Any ideas for something that might be a tad harder than this, but not so hard that a bunch of sophomores with a week's worth of crypto lectures couldn't solve it in a reasonable amount of time?
EDIT: To clarify, I'm looking for hash functions which are not cryptographically secure, and can be easily inverted (or at the very least caused to collide) without brute-force search of the input space. As a general rule, hash functions which you could even mistake for cryptographically secure are probably too hard to break for this assignment.