Answer
Conjugate acids of weak bases like $N{H_4}^+$ and $CH_3N{H_3}^+$ act as weak acids. Other type of cations that also act as weak acids are small, highly charged metals, like $Fe^{3+}$ and $Al^{3+}$.
Work Step by Step
Any cation that is the conjugate acid of a weak base can act as a weak acid according to the generic equation:
$$BH^+(aq) + H_2O(l) \leftrightharpoons H_3O^+(aq) + B(aq)$$
Small, highly charged metals react with water to become hydrated, and then act as Bronsted-Lowry acids. Example:
$$Fe(H_2O{)_6}^{3+} + H_2O (l) \leftrightharpoons Fe(H_2O)_5(OH)^{2+}(aq) + H_3O^+(aq)$$
