One of the old GURPS Time Travel adventures (A Nile Elation; involving an atomic bomb planted in the Great Pyramid millennia in the past that would detonate in 1973) has a section involving Napoleon's visit to the pyramid and him being hypnotically (well sort of) used to arm the bomb.
Then there's the ever popular Napoleon gets his hand on advanced tech (specifically I'm thinking of "Napoleon's B52":
www.dalecozort.com/AHNewsletter/Apr00/napoleon1.htm).
Other possibilities:
1. interfere with Napoleon's wedding to Josephine in 1796, possibly using it to attempt an assassination
2. prevent the suppression of the mutiny at Spithead in 1797, weakening the British navy
3. use a time travelling mini-sub to attach bombs to the warships of Nelson and Jervis prior to the battle of Cape St. Vincent
4. Assassinate pope Pius VI when he's attempting to flee Rome in 1798 ahead of the French forces
5. Assassinate Ferdinand IV of Naples before his declaration of war on France
6. Destroy or steal the Rosetta stone at the time of it's discovery (1799)
7. Prevent Napoleon's forces crossing the Great St. Bernard using explosives or other methods
8. Interfere with, for good or ill, with Volta's experiments with electricity
9. Assist the rebels against French forces suppressing the slave rebellion in Santo Domingo led by Toussaint-L'Ouverture
10. Stranded time traveller interferes, accidentally or otherwise, with the birth of American author Nathaniel Hawthorne (1804)
11. Something weird happens at or just before Napoleon's coronation in 1804
12. The duel between US politicians Alexander Hamilton and Aaron Burr in 1804 has numerous possibilities
13. Napoleon uses Fulton's steamboats or submarines to attack the British navy, possibly thanks to some extra-temporal advice
14. There's the matter of Louis Charles Capet, the disappearing Dauphin
15. Something happens to Beethoven or Goethe
16. The Chesapeake-Leopard Incident between the USA and Britain in 1807 leads to war, who's egging things on?
17. A person accidentally dumped in 1809 (time vortex? wake from Dalek time machine? TARDIS stowaway?) interferes with the birth of: Alfred Lord Tennyson, Edgar Allan Poe, Felix Mendelssohn, Charles Darwin, Abraham Lincoln.
18. A perennial favourite; in 1809 the British diplomat Benjamin Bathurst disappeared under mysterious circumstances. What happened? Was he dropped into a parallel universe?
19. The madness of King George III. Was it natural in origin or part of a Sinister Plot (TM).
20. The Luddites have appeared in Who, along with the Master and the Rani but 1811 has other possibilities for slowing or disrupting the industrial revolution in the UK.
21. William Henry Harrison, the shortest serving president of the USA (and probably the most stupid) was involved in the battle against Native Americans, what if he was killed there?
22. On Barbados there were a number of Weird Happenings involving coffins belonging to the Chase family in a sealed tomb in 1812. Each time the tomb is unsealed for an internment, the coffins are
found rearranged. Hmmmmmm.
23. 1828 the mysterious appearance of Kaspar Hauser
24. What happens if General Claude Francois Malet conspiracy against Napoleon during his absence in Russia succeeds?
25. Sören Kierkegaard, the Danish philosopher, was born in 1813. Or was he?
26. The US-UK War of 1812 has many possibilities, especially the Battle of New Orleans.
27. The (supposed) death of Marshall Ney has possibilities; Louis XVII ordered his death but was the firing squad staged? According to Quentin Dick (Irish MP) the body was carried away in a
carriage within three minutes, without medical examination. Did the old soldier end up in North Carolina?
28. 1818: the birth of one Karl Marx and Mary Wollstonecraft Shelley writes
Frankenstein
29. The future Queen Victoria of Britain was born in 1819, unless someone stops it.
30. The great Cholera pandemic began in 1826 in India, spreads from Russia into central Europe
by 1831, and reaches Scotland by 1832. Was it a natural event? Bonus points for making your PCs stand by (or assist) in many deaths.
31. 1830, Charles Babbage and his Analytical Engine, did someone from the future give him a few pointers?
32. And finally, 1831 the Virginia slave revolt led by Nat Turner, 55 whites die.
Hope this helps.