Sequential numbers to unique-looking numbers
Not sure what you mean by &quot;totally obvious&quot;. Technically, every permutation of $\{1,\dots,10^8\}$ would work here.
You can randomly generate such a permutation, store it in an array, and use this array as an index.
If you want a specific example, which may look at first glance non-sequential, take some big number $p$ such that $\gcd(p,10^8)=1$ (e.g. $p= 19683$), then the id of user $i$ will be $i \cdot p \pmod{10^8}$. This is guaranteed to be unique for each user (up to $10^8$ users).
It may look random only at a glance, but anyone who takes enough time can figure out this is not random.