Hilly, hows about a pyramid session, something like (adjust to your level)
Warm up (400m?)
Main set:
8x25 sprint (hard as poss)
4x50 slightly less hard
2x100 steady
1x200 steady, slower
2x100
4x50
8x25
Swim down (200, concentrating on form, or even just doing slow drills)
That kinda thing anyway. Incorporates really fast tough stuff and endurance. Easy to remember, and simple for starters.