Take child to school, go to the beach and collect shells and return in time to pick child up (this is my life, except instead of collecting shells I'm making silly computer games). I play this a few times...
First play - went straight to the car and didn't get any shells
Second - got too many shells and was late
Third - wanted to see how big the beach was or if I could reach the sea - was late
Fourth - collected a few shells, was late
Fith - Collected 2 lots of Shells and won.
Very simple but you could add lots more to make it more interesting, a few ideas would be a HUD to show the time so you can make decisions on far to travel to leave you enough time to get back and an incentive to push the time limit by having a score that relates to how many shells you collect. Also you could add obstcles on the beach like sunbathers, dog walkers etc cause you to lose shells or slow you down.