I am looking for the best ones and later you will find activities for each of them, meanwhile you can watch them and repeat the words.
1.- Here is a video with a general explanation of the sounds from the vowel u.
2.- Exercises with the vowel /oo/
3.- Vowel sounds
3.1 Listen to the sounds here http://international.ouc.bc.ca/pronunciation/minpair12.html
3.2 Dictation practice http://international.ouc.bc.ca/pronunciation/dictation12.htm