If you've got a little money and a little bit of time, the world (of entertaining your kids) is your oyster. Even though these fall under the category of requiring money, some are very low cost or may only require things you already have on hand.
Make Mushy Play Stuff
There's so many fun things to for kids to stick their hands in. If you have a little time to make some of these, they can keep kids entertained for hours. Here's a variety of ones depending on what you have on hand and what you have the tolerance for:
Cloud dough - The basic recipe is just flour and oil, though there are some cool variations at this link as well.
Oobleck - The classic Dr Seuss named "slime" made of cornstarch and water
Salt dough - The difference between salt dough and play dough is that you can shape salt dough and bake in the oven at a low temperature to preserve what you've made - ornaments, candle holders and other decorative items.
There's so many fun things you can bake with kids but they do usually need supervision. There are lots of good kids baking books and here's a list of recipes, if you don''t want to wait for a book to come.
This kit Scientific Explorer My First Mind Blowing Science Kit has proven to be very entertaining for our daughter, if somewhat complicated (parent or older kid supervision definitely required).
We also have lots of nature suggestions on the No Money, No Time page but here are some items that cost a bit more and take a bit more supervision.
Geodes - If you have a kid that enjoys sparkly thing, they'll love cracking open geodes. The more you let your kid do the work, the more time this will take up.
Field Guide - if you just want to get out and explore, Backpack Explorer: On the Nature Trail: What Will You Find? is a great way to have kids identify what they're seeing.
I wish I had time to write up everything but, I don't (at least not yet, let's see how long this lasts). So here's a lot of other suggests with links when needed: