There are a lot of apps available, just search on mental math. I've found that relatively simple arithmetic done repeatedly works well, look for those kinds of apps. About 10-15 mins of these problems per day is enough.

Stupid question, wouldn't coding (keeping the flow of the code in mind), being the same type if exercise?

No it doesn't give me the same effect, surprisingly. However it does definitely help with coding.

It gives your working memory and symbol manipulation a workout. Coding uses these facilities, but might not work them directly enough to be as good at training them. That's my theory so far.

For me coding is mostly about memory, math is more about processing.