Translation:We did a lot of things in the spring.
They can be related, but they are not really interchangeable. Cosa is just "thing". "Algo" as a noun would be "something". So you can have many things, but you can't have many somethings. And there can be "una cosa importante"--an important thing--but that's a bit different from "algo importante"--something important.
I have a question for native Spanish speakers. How common is it in your area to say "hicimos muchas cosas"? Most of the Spanish speakers I know would probably say "Hicimos mucho en la primavera." I've noticed that they don't use "things" in an abstract way as often as we do in English. What do you think?