Combat

I assume you mean for festival plaza right?  Well in that case...

Fortune tellers give you boosts to FC gain.  Higher stars mean better chances for larger boosts.

Treasure Hunt (a specific type of Lottery Shop) can give you bottle caps and gold bottle caps.  Higher stars mean higher chances of getting those.  It's also worth noting that your first use of one is a guaranteed bottle cap.

If you have two Five-star Rare Kitchens, you can make one Pokemon go from level 66 to 98 by buying the two most expensive meals from each.  It has to be exactly level 66 to work.

Five Star General Goods sells PP ups.

Five star dye houses let you use berries to dye white clothing items any one of 8 colors in "Pastel" and "Dark" variations.  There's also a "Bright" variation but you can only get that in the color that the dye house is (for example a team purple dye house will only give you bright purple).

Pretty much everything else is a waste of your time and friend coins.

Thank you!