Get outside and get some excercise. Ride bikes, play a game, or just walk or run around the neighborhood.
Step2
Go to a nearby playground or park. If it's a special day, take them to Family Fun Center or something.
Step3
Afterwards, play with your kid's favorite toys. For example, Legos, Transformers, GI Joe's, etc. Even if it means you have to actually pretend to *be* Optimus Prime and run around the house getting Decepticons.
Step4
Pick one TV show to watch. Don't spend the whole day watching TV. 30-60 minutes is plenty.
Step5
Try at least once to spend 15 minutes or so telling your kid something you wish your parents would have told you.
Step6
Work on some kind of skill with your kid, like throwing a ball, or riding a bike.
Tips & Warnings
If your wife is away for the day, try to do something around the house like a load of laundry or dishes.
Use events that come up during the day to teach your kids valuable life lessions, like telling the truth or doing things the right way.
Don't put off spending time with your kids for too long. Eventually, they won't want to spend time with you and it will be too late.
Don't use money to make your kid happy. Otherwise they'll turn into a brat and will want you to spend money on them every time you spend the day together.