After getting a stun, I tend to take advantage of Mai's forward crouch walk animation in order to take a glance at what the opponent "might" try (i.e. seeing if they are going to hold so that you can buffer a 236P or 2T for a hi-counter. Otherwise, you can go for a mid punch, mid kick, or a high launch for a juggle if you have no intention on playing the stun game). It might be wise to utilize 1K (or 6PK~into whatever follow up due to 6P being i11) if you know the opponent is going to press buttons. 2K+H is surprisingly useful (mainly against low/mid intermediate players, partially due to it's range and its high crush property. It is slow and somewhat telegraphed however, even used in a string. The issue is that PP also has a mid punch follow up and the player might be forced to guess the follow up from PP). It also combos into P (Mai is free to do a few follow ups but she can also take advantage of opponents who are obsessed with the hold function, especially after using 2H+K). Best not to over rely on it too much and be sure to space it just right to make it difficult to deal with. 2K and the follow ups from it are also very useful because 2KP registers as combo (the opponent can hold low if 2KP is used by itself outside of the stun) and 2KP+K is a mid follow up (this is obvious, but Mai is noted to have quite a few useful high crushes against opponents using too much highs).
@DestructionBomb mentioned that 66K is particularly useful. This is correct for a large number of reasons. Because, if opponents like to whiff (for some reason) a lot at the midrange, Mai can readily stun them (this also counts 6K+H is another useful way of "going in" or "moving forward" on the opponent). I also find this useful in stun (along with side step, when used offensively during the opponents stun state to bait out a hold or to push the critical threshold a bit further). The reason why 66K may be useful in stun is because of its delayed startup animation (which would look weird from the opponent's perspective because she also has 66P which also has a rather suspicious startup animation, forcing the opponent into a rather bad situation).
Plus, there are too many ways that Mai can space and turtle against the opponent, based on how her move pool is set up: She has a lot of annoying lows that can be used from a distance. Plus she has the (albeit, deceivingly useful) Kachousen, 4K, 6K, 3K, and 6P (plus follow ups if the opponent is getting a little "too" obnoxious), along with 7P and 9P to implement additional mobility options (Mai can outright discourage of the use of WU kicks, especially if opponents happen to be stuck towards the wall).
There will be more to come later. Some of guys ought to take a look at Mai in some of the other KOF titles, especially XIV!