What are some examples of when a mixture can always be a solution?
All homogeneous mixtures
"All solutions are mixtures, but not all mixtures are solutions."
A mixture is a solution only when it is homogeneous; i.e. the components are dispersed evenly throughout.
Some examples of solutions are
sea water (various dissolved ions)
#"CuCl"_2#solution (dissolved #"Cu"^(2+)#and #"Cl"^-#ions)
vinegar (contains acetic acid,
hydrochloric acid (concentrated
coffee (dissolved sugar molecules)
regular tap and bottled water! (there are, albeit in small quantities, some dissolved ions in tap water and bottled water (check the information label); this allows for the water to conduct a slight current, which is why you were probably told not to bring electrical devices in the bathtub!)