The Natural History Museum admission is free the first Tuesday of each month. That's tomorrow, for anyone who doesn't have to work and doesn't mind the crowds on the free museum days, and it will save you $8/person (unless there's an extra charge for the special exhibition).
The Machu Picchu exhibition should be really interesting to see.
LA Museum free days (as of March 2003)
Arboretum -- 3rd Tuesday
Autry M of Western Heritage-- Thursdays 4-8PM, 2nd Tuesday
Craft and Folk Art M -- 1st Wed
Huntington Library, Art, and Gardens -- 1st Thursday
Japanese American National M-- Thursday 5-8PM, 3rd Thursday
Long Beach M of Art -- 1st Friday
LA County M of Art (LACMA) -- 2nd Tuesday
M of Contemorary Art (MOCA) -- Thursdays
Natural History M of LA County -- 1st Tuesday
Pacific Asia M -- 3rd Saturday
Page M at La Brea Tar Pits -- 1st Tuesday
UCLA Fowler M of Cultural History -- unsure: Thursdays, Thursdays & Sundays, or every day
UCLA Armand Hammer M -- Thursdays
Free everyday:
J. Paul Getty, if it's still open
Getty Center
CA Science Center