One of my favorite drinks, especially in the summer, is a coffee tonic. At base it's just coffee plus tonic water, so they're very easy to make while giving you plenty of room to explore.

A cold glass of light-brown coffee tonic, with ice.

Simple Coffee Tonic Recipe

Here's the base recipe I use:

4oz (120ml) cold coffee
2oz (60ml) tonic water
1/2oz (15ml) simple syrup
ice

Coffee tonic ingredients: a 1-pint mason jar containing simple syrup, a 1-quart mason jar of cold brew coffee, a small bottle of Fever-Tree Tonic, and a glass of prepared coffee tonic with ice.

You can adjust those ratios to taste. You'll likely need different ratios depending on your coffee, brand of tonic water, and the sweetness of your simple syrup.

I typically use a mild cold brewed coffee, though I've also gotten good results with regular hot-brew (cooled down to room temp). Espresso diluted over ice would probably also be great.

It's hard to beat Fever Tree tonic. While other tonics also give decent results, my coffee tonics are way better when I splurge on the good stuff.

Tonic water has a lot of sugar in it already, so I mostly add simple syrup because I really like sweets, and so that I can add other flavors. I try to keep cardamom simple syrup on hand for my coffee tonics.