+1 for the exhibition at the Barbican (be warned it can be a bit of a maze)
Would also look at the walkie-talkie gardens (free but you have to book)
If you've got money to spare, there's also the shard viewing gallery.
Apparently there's an Alien:Covenant live action type experience at Tussauds - way back in the 90s there was something similar, Alien War which was good fun, not sure how this compares though.