There's always a way that you can make your weekend fun for the kids and still have relax time for you if you're absolutely buggered from working all week.
1) The Play-date Option - the weekend is a great opportunity for children to get together with friends and have a play date. Call up a few friends and ask if the kids can go over and play for the afternoon on Saturday. If they say yes, then offer to drive them over. You can sleep, do some housework or watch TV while they're out playing, then of course offer to pick them up.
2) Relax with friends - Kind of like the idea mentioned above, however if you have friends with kids, why not all get together for a small BBQ, the kids can play and have fun, and the adults can sit back, talk and relax.
3) An outing on a Sunday - Keep Friday and Saturday for doing nothing, and then on Sunday take the kids somewhere fun. Choose carefully, if you don't want to do a lot of walking and carrying things then avoid places like the zoo, the aquarium or amusement parks.
Stick to things more local and stuff that kids like doing such as bowling, the cinema, or a live show.
4) Let community centers be your friend - There are usually a lot of free, and sometimes paid activities your children can do on the weekend. It gets them out of the house and doing something fun and creative. Community Centre's might have a craft hour on a Saturday, or the local library might have story time in the morning. Check out your local one.
5) Do fun stuff at home - Bring out the old fashioned stuff like board games, funny movies and pencil and paper. Do fun stuff like that at home with the kids, its enjoyable for them and doesn't require a lot of effort at all.
These ideas are based for families who work full time and find it hard going out on the weekends and doing fun stuff with the kids.