A backyard projector and inflatable screen turn your lawn into a theater for years if you buy quality gear once. Don't waste money on cheap components that will break after one season of humidity and dust.
Wait for the end of summer when stores clear out inventory to make room for indoor gear. Look for the previous year's model, as outdoor tech rarely changes enough to justify paying the premium for the newest release.
The 18-month window is a deadline, not free money. If you have a remaining balance when those 18 months end, you will start paying interest on whatever is left over.