This is a warmup activity that has worked quite well for me and my intermediate/upper-intermediate classes. It encourages students to speak out early in the class, builds up their confidence creating sentences with a limited vocabulary, but also motivates them to learn more. Let me know what you think of it.
(Alex, Guangdong province, China)

1

Show or tell your students this news story (http://www.bbc.co.uk/news/magazine-12894638) about [England soccer manager] Fabio Capello's belief that he only needs to know 100 English words in order to do his job. Ask students whether they think he's right - "Do you only need to know 100 words to get by in English?"

2

On a handout or the OHP, show your students the 100 most common English words, as compiled by the Oxford English Corpus (the list is reproduced below, in rank order and alphabetical order). Ask them, in groups or as a class, to try to make simple sentences using only these words. I usually ask a student to call out a word, then ask another to suggest a word to go before or after it. This continues until a grammatical (not necessarilly logical or true) sentence has emerged.

3

For more advanced classes, try to compose a dialogue. ("What do you want to do?", "I would like to go back to where I come from", "Where do you come from?" "Over there", etc)

4

Congratulate your students every time they put together a grammatical sentence, and try to tackle any problems that emerge. The absence of "is", "are", "was" and "were" is usually an obstacle, but can be got round by shifting the sentence into the future or conditional tense.

5

Ask your students the question again: Can you get by in English knowing just 100 words? Generally, classes conclude that you can if you only need to say basic things, or if what you're saying doesn't have to be true. Anything more complicated, or accurate, is impossible.

6

End by making the (highly motivating) point that English actually becomes easier the more vocabulary you learn, because you've got more ways of getting your point across.

-- Ranked List --

1. the
2. be
3. to
4. of
5. and
6. a
7. in
8. that
9. have
10. I
11. it
12. for
13. not
14. on
15. with
16. he
17. as
18. you
19. do
20. at
21. this
22. but
23. his
24. by
25. from
26. they
27. we
28. say
29. her
30. she
31. or
32. an
33. will
34. my
35. one
36. all
37. would
38. there
39. their
40. what
41. so
42. up
43. out
44. if
45. about
46. who
47. get
48. which
49. go
50. me
51. when
52. make
53. can
54. like
55. time
56. no
57. just
58. him
59. know
60. take
61. people
62. into
63. year
64. your
65. good
66. some
67. could
68. them
69. see
70. other
71. than
72. then
73. now
74. look
75. only
76. come
77. its
78. over
79. think
80. also
81. back
82. after
83. use
84. two
85. how
86. our
87. work
88. first
89. well
90. way
91. even
92. new
93. want
94. because
95. any
96. these
97. give
98. day
99. most
100. us

-- Alphabetical List --

a, about, after, all, also, an, and, any, as, at, back, be, because, but, by, can, come, could, day, do, even, first, for, from, get, give, go, good, have, he, her, him, his, how, I, if, in, into, it, its, just, know, like, look, make, me, most, my, new, no, not, now, of, on, one, only, or, other, our, out, over, people, say, see, she, so, some, take, than, that, the, their, them, then, there, these, they, think, this, time, to, two, up, us, use, want, way, we, well, what, when, which, who,will, with, work, would, year, you, your