I feel that fully half of what makes the Pimm's Cup such a beguiling summer cocktail is in the garnish. There are multiple variants, of course, but I favor the ultra-simple slice of cucumber + slice of lemon.
Pimm's makes a variety of styles, and that namesake cocktail made with the formulation known as No. 1 has traditionally been popular in the south of England, appearing as one of two staple drinks (the other sip of choice would be champagne) at such rarefied events as Wimbledon, the Henley Royal Regatta and the Glyndebourne opera festival.
Knowing all that, it's interesting to see that the recipe for the classic Pimm's Cup cocktail is terrifyingly simple. Common, even...
2 oz Pimm's No. 1
4 to 6 oz lemonade (some use lemon/lime soda; I favor ginger ale)
Mint leaves, and slices of lemon (or orange, strawberry, apple...)
Originally, the cocktail required borage leaves in lieu of mint/cucumber, but as borage is a bit tough to come by in U.S. markets, cucumber is the go-to garnish hereabouts.
But as I mentioned, I find the cucumber/lemon combo to be particularly magical. The cooling qualities of the cucumber alongside the citrus zip and vigor of the lemon go a long way in gin-style cocktails (and Pimm's No. 1 is one such blend) in particular, since gin is, by nature, herbaceous.
I've even become a great fan of lemon and cucumber slices served with water. So simple, but the scent and flavor results are elegant... perfect for brunch, for time spent on the deck/patio/fire escape and for adding a touch of class to your next grill-fest. Give it a try and see if you don't become a convert.