My daughter's waist has the circumference of a bamboo stalk, so I'm always on the lookout for pants with adjustable waistlines or elastic. Donna's hack retrofits any pair of pants with belt loops, and it's a cinch to do.
Get it? A cinch? Oh, how I amuse myself.
Upon leaving on vacation and forgetting to pack a belt, I found a quick, easy, and cheap way to fix pants or shorts that are a bit big.
Cut elastic to the length you need, loop it around two side belt loops, pull tight, and tie using a double knot.
I've done this with my kids' pants as well as on my own. I will just leave the elastic on until it's no longer needed, then snip it off with a pair of scissors. A t-shirt will cover the elastic and no one will be the wiser.
Elastic comes in various widths and colors. I get the 1/4-1/2 inch width. I always keep some in the car in my supply kit.
Related: How to tighten pants without sewing? Talk amongst yourselves. (Lots more ideas in the comments of this post.)