"What's in your spare parts closet?
(I promise I won't tell your spouse)"lol, my spouse already knows, and i have years worth of an accumulation of spare parts............
anything extra for your needs and hive setups are really invaluable, deeps, supers, frames, bottom boards, inner cover, etc......you can switch things out, add without worrying about running out of equipment. also for swarms and divides extra equipment, even nuc boxes are great.
bakers mentioned the bucket pails, and shims.... i have extra of these, so i can take the empty off and flop the new one right away to replace it. shims can take a beating over time so i keep extra on hand. i also cover feed with old deeps not suitable for brood boxes anymore. (you can probably buy these old beaters cheap). i have, and actually some i fixed up to use and are still in service.
oh and one more thing..........extra hive tools........cuz you will lose them and find them next year.........maybe!...........