I'm thankful for your concern for your grandmother; I'm afraid some people in her situation never have any visitors or see anyone who cares about them. Often, while visiting someone in a nursing home the staff has asked me to spend time with residents who never have a visitor (which I've been glad to do).
Don't feel guilty because you can't do everything you'd like to do for your grandmother. The next time you visit, however, talk with the staff and find out what activities they have for residents (including chapel services). Then encourage her to take advantage of them and be as active and friendly as possible.
Your pastor also may know a church in her area that has a special outreach to local nursing homes. Many churches do this, and others should be encouraged to reach out to the elderly. The Bible commands us to "Give proper recognition to those widows who are really in need" (1 Timothy 5:3).
In addition, pray for your grandmother, and let her know you're praying for her. Pray that she will have a special sense of God's presence and peace during this stage of her life, and that her faith and hope will be in Christ. Ask God also to help you think of other ways you can express your love for her. Nothing, for example, brightens a lonely person's day like an unexpected phone call or a cheery card.