Then don't activate cellular during the trial period. Buy a cellular version, if you're confident you'll actually need it... then activate the cellular service when you're convinced you want to keep the Watch.What about the fee to activate the Apple Watch on your cell phone provider and the monthly fee ?
I can't promise you'll decide to keep the Watch, but I doubt cellular service will be the feature that tips the balance in favor of keeping the Watch. If you don't find the Watch useful for all it's other features, having cellular is unlikely to change your mind.