What kind of mechanics can make MTG fun for my opponents?
This isn't mechanics specific, but theming is a good way to go. Themes can be based on Magic blocks with poor synergy or underpowered mechanics, like Kamigawa block (which likely will just make people laugh), or themes can be pulled from outside of the game, like trying to make a Lion King themed deck (or something else you like). Themes limit your options while being interesting to watch, especially if other players know the theme. Half the fun of out of Magic themes is in players trying to guess how specific cards fit into the theme.

As an example, I'm currently working on an Apocolypse deck (not the block, the out of Magic concept). The commander will be  Child of Alara, with the four horsemen of the apocalypse as  Iroas, God of Victory (War),  Mogis, God of Slaughter (Death),  Phiraka, God of Affliction (Pestilence), and  Phenax, God of Deception (Famine). After getting these five cards set, the next step of the deck was to find cards that included the name of the four horsemen, so things like  Sudden Death and  War Elemental. This doesn't quite fill up the deck, especially if you're avoided hated cards like  Death Cloud, so the rest of the deck is made up of mechanics that fit the four horsemen's mechanics. This included some mill for Famine, like  Glimpse the Unthinkable, and some extra sacrifice for Death like  Cruel Edict.

What you'll notice is that this deck isn't very synergistic and it will play differently every game. Competitive decks are tuned to play identically every time even though you only have 1 of every card. The theming will take away that consistency and will be fun to watch. And generally, you're going to have to put a decent amount of thought into the cards in a theme deck to make sure you're staying on theme, so it's going to take as much thought to produce as a well-tuned competitive deck. This process has the added benefit of causing you to think about Magic in a way you wouldn't normally, and maybe you'll find some playstyle or set of interactions you hadn't ever done before.