That really isn't a feasible plan
Let's assume that deliberately trying to lose doesn't itself violate the magical contract; seems unlikely, considering the furor when our muggle athletes deliberately lose, but for the sake of argument.
The circumstances surrounding the tasks made non-participation difficult:
- The first task is an angry dragon which, as far as Harry knows, will not be subdued until he retrieves the Egg. It might be argued that he could have simply avoided it until the Tournament organizers got bored, but that seems like a terrible plan: prolonging the confrontation make it more likely that he will be horribly injured, which just isn't worth it.
Harry dramatically misunderstood the nature of the second task, which in any case was designed to provide some "incentive" for the champions:
Harry Potter has to go into the lake and find his Wheezy -"
"Your Wheezy, sir, your Wheezy-Wheezy who is giving Dobby his sweater!" Dobby plucked at the shrunken maroon sweater he was now wearing over his shorts.
"What?" Harry gasped. "They've got... they've got Ron?"
"Harry, you prat," said Ron, "you didn't take that song thing seriously, did you? Dumbledore wouldn't have let any of us drown!"
Goblet of Fire Chapter 26: "The Second Task"
For the third task, Harry obviously had to at least attempt the maze; doing otherwise is pretty blatantly refusing to compete. However, Harry learned ahead of time that the maze would be full of dangerous hazards (and Harry knows, better than anyone except for Cedric, just how dangerous):
The third task's really very straightforward. The Triwizard Cup will be placed in the center of the maze. The first champion to touch it will receive full marks."
"We semply 'ave to get through the maze?" said Fleur.
"There will be obstacles," said Bagman happily, bouncing on the balls of his feet. "Hagrid is providing a number of creatures... then there will be spells that must be broken... all that sort of thing, you know.
Goblet of Fire Chapter 28: "The Madness of Mr. Crouch"
He could waste time by wandering aimlessly through the maze, but why would he want to? It's the same argument as the dragon: the quickest (and safest) way to survive is to finish the task as quickly as possible.
Of course, it doesn't hurt that he's started to get a little invested by this point:
He'd got this far, hadn’t he? What if he actually managed to win? Fleetingly, and for the first time since he’d found himself champion, he saw again that image of himself, raising the Triwizard Cup in front of the rest of the school...
Goblet of Fire Chapter 31: "The Third Task"