The slave-boson mean-field method is used to study the two-dimensional Hubbard model. A magnetic phase diagram allowing for paramagnetism, weak and strong ferromagnetism and antiferromagnetism is constructed and compared to the corresponding phase diagram using the Hartree-Fock approximation (HFA). Magnetically ordered regions are reduced by a factor of about three along both the t/U and density axes compared to the HFA. Using the spin-rotation-invariant formulation of the slave-boson method the helicity modulus is computed and for half filling is found practically to coincide with that found using variational Monte Carlo calculations with the Gutzwiller wave function. Off half filling the results can be used to compare with quantum Monte Carlo calculations of the effective hopping parameter. Contrary to the case of half filling, the slave-boson approach is seen to greatly improve the results of the HFA when off half filling.