And finally, one I've actually used!
Which I stole from Russell T Davies.
To see how it went, read the Actual Play! And if you're more interested in the story, avoid reading past this cut beforehand to avoid spoilers.
The Hammer Of Time
The travellers arrive in 21st Century Earth to find two space armadas fighting - future humans and Torlan warriors, who have come back in time to destroy Earth centuries before the war starts.
Inside the command ships, both sides have proud, hawkish commanders, but also people on both sides being killed and maimed in this final, desperate battle. The Torlan warlord wants to destroy humanity, while the future humans will see that this might be a problem and their admiral might be more willing to help. If not, fighters or civilians on either side might be more ready to assist.
Both sides have anti-laser defences they can use in case of boarding and in their short-range space fighters, which could be repurposed to force a ceasefire...
Things To Do: Save the Earth, and if at all possible stop this happening again.
Antagonists: Torlan warriors, and possibly future human soldiers as well.
Action Scenes: Escaping the middle of a space battle. Sabotaging the command ships of two armadas. Stopping the world-cracker drill on modern Earth. Shutting down an interplanetary war.
Visuals: Massive space battles with the travellers right in the middle. Their ship getting knocked around and clipped by passing fighters - taking a space battle and making it funny. Seven-foot alien warriors guarding a hole in the ground in a modern suburban street. Space fighters pursuing each other around modern London.
Problems: There's a lot of ground for the travellers to cover, with two warring factions to deal with and a planet full of innocent bystanders in danger.
Things To Prepare For: Maps of the flagships might be useful, and Torlan stats might be as well.
Continuing the Adventure: Where did the Torlan get the time-travel technology from?