Theater and music events can be particularly enjoyable for those with a mental disability, especially those who are very high functioning. Many theater and classical music companies have education programs that will allow you to purchase curriculum based on their programming. This can help prepare those who are about to be seeing a presentation with everything from proper theater etiquette to specifics about the piece itself.
Those who are quite strong physically may benefit from sporting events. Those who are not as mobile may also find it enjoyable to participate in events such as swimming, table tennis or wheelchair basketball. Impromptu "pick-up" games of popular sports such as basketball, soccer and baseball can be enjoyed by those with mental disabilities and will increase their fitness and help them stay in shape.
Like theater and symphonic companies, most museums have special education programs. Call the museum you are interested in visiting beforehand and talk to them about the appropriateness of the museum with a group of mentally disabled people. The museum may be able to include some private activities for the group or give an idea about which exhibits will be best for your group's needs. Exhibits that are highly interactive may be more interesting for those with mental disabilities.
Those with mental disabilities often enjoy arts and crafts, just like any other student. Depending on their cognitive function, simple crafts like coloring or making handprints with paint on paper or fabric can be fulfilling. Higher functioning students may enjoy more complex crafts such as braiding lanyards, making jewelry for themselves and family with jewelry-making kits, painting a mural together or sewing clothing for themselves or dolls.