I feel like me and my fiance are not doing enough stuff together lately. I dont want us to slip to far apart and then not be able to fix anything. Any advice on how and what we can do to bring us closer together?
Unless you think it entirely inappropriate, ask him to make you a promise (which you also make in return) to be honest enough to tell you if he is losing interest in the relationship because you don't want acrimony/pain and that it would be better to separate amicably than not, if indeed that is what one of you wants (only one of you needed for this because that is terminal).
We accept that not all things we do will be of interest to each other so there is no point forcing it. As long as you share the important things that should be ok. If you try and push your interests you may lose out.