To invite an NPC, you must be dating them, but not married or engaged to them. Call them on the mobile phone and select "Invite", and you should see Balloon Ride available to click.
The event is based on the vanilla balloon ride event, but has several sets of unique dialogue for each of the vanilla datable NPCs.
Finishing the event gives the player 75 friendship points (3/10 of a heart) with the accompanying NPC.
This mod is meant to showcase the new mobile phone Invite feature. If you are interested in creating a similar custom event, please use this as an example, and read the documentation here:
https://www.nexusmods.com/stardewvalley/articles/481
This mod is compatible with Stardew Valley Expanded (it has two versions in it to account for SVE) but still only available for vanilla NPCs.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.