It depends on what your priorities are. Changing cleaning products is a good one. Or using less. Most people use too much shampoo, for example.
Investing in things that you will use for YEARS is another. For example, a good set of cloth diapers can last you several children so the initial investment pays off.
I did buy a new car and at the time I crunched the numbers. Due to the interest rate on a used car, I was able to get a much better deal on the new car, had a warranty, etc. I chose carefully and have driven that same car for 10 years and it was paid off well before the end of the loan. So while on the surface things like "don't buy a new car" sound good, do the math before you cross it off. I bought the middle of the line of my car - one I could afford but had a few more bells than the lowest end. I don't buy a car with the idea that I'll sell it in a few years.
I love going to flea markets and thrift stores. I think they're fun. I buy books and CDs at the friends of the library sale because 1. they're inexpensive and often in great shape and 2. it supports a resource I love.
You could make your own grocery bags out of old clothes or fabric scraps. Locally they now charge you per bag if you don't bring your own. We've long had a set of our own bags, but if you want ones that are more fun, build your own set.
Things like growing your own garden we do, but we're not sure we really get supercheap veggies, with the heat around here and the water bill. DH does use rain barrels to water but sometimes there hasn't been enough rain to fill them. We also compost so we never use commercial fertilizer.
Even a simple thing like shopping off a list vs off the top of your head or using one of those scanners so you SEE your total and maybe don't buy that extra whatever or buy in bulk when it's a good deal (compare price per unit) can help without making you feel like you need to rinse and reuse ziplock baggies.