1. Make up games
For example, the tape measure game. This involves trying to extend a tape measure as far as possible up into the air without the lecturer noticing. My current record is 2m 45cm.
2. Write notes
This one requires the company of interesting people. Failing that, you can conduct polls of your fellow students, eg most boring lecturer, why are the fishies dead, am I mad etc.
Especially recomended for lectures in the maths department, where it tends to be warm. If you're going to sleep, it's always a good idea to have someone to poke you if you snore and protect you from people throwing paper at you. Also, don't wake up if someone says 'finished', they may be referring to a question and not the lecture.
4. Write a book
5. Come up with escape plans
Unless you're Dan, in which case actually escape.
6. Catch up with correspondence
Not everyone is modern and up-to-date, and sometimes people may insist on sending you bits of paper with stuff written on, known as letters. Lectures are a good time to deal with these, as lecturers often can't tell whether paper with stuff written on is lecture notes or not.
7. Take notes
Only recommended as a last resort, or if the lecturer is growing suspicious.