One of mine is shopping for clothes in the kids' section. I'm a smaller adult and can fit into L/XL stuff from the girls section for some basics (undergarments, plain shirts, socks, shoes...) and even the boy's section! Boys is cheapest, girls is next, women's clothes are 10x more expensive for some reason and I've even found the exact same items in all 3 departments!! Like silk long johns, for example. They were $44 in the women's section (size XS), $24 in the girls' section (size XL) and $14 in the boys' section (size L). Same brand/color/style!
Another is when we are out & about and want a treat, instead of paying $3-4+ each for an ice cream cone at the ice cream shop, we stop at a grocery store and get ice cream there instead. Sometimes we split a big container, other times we get the mini containers or ice cream bars, and it ends up being around $1 a person instead.
Would love to hear some of your obscure tips!