I guess your kids are at international school, otherwise you don't have to worry about filling your day when your kids are at school really, as they'll be home before lunch in the state system :)
I think if you are going to have whole days to fill study would be a good option - if your German is already fluent then the OU degree or a masters or PhD or whatever your level (higher degrees might be an option in English via a local university) would certainly be an option if you can fund it ...
Teaching is the eternal option work wise but bits and bobs of language teaching won't earn lots and can be frustrating for one reason or another. If you have other skills of course you can teach other things, not just English, for example via VHS, or take courses to learn all kinds of obscure things from martial arts to a totally new language to crafts etc. etc.
Write a book? You can answer "what do your do?" with "I'm an author" even if you don't get published ;)
Write freelance articles and send them to local papers in the UK and see if anyone will publish - you might not make any money but you can then say you are a travel writer or journalist?
Join an international women's group - if you have time to go to their lectures etc. you might make contacts and get some ideas...
You can volunteer for Caritas I think, doing practical stuff...