I'm not sure what stage of Dementia your participants are in, but I work in a memory care center for mild to moderate memory loss. I've found in my 2.5 years there that you can do a lot of the same activities that you would do with other seniors, but you have to remove a few steps. For example, we have the game Loaded Questions and we just play with the cards, not the board. Many of them also like things like personality tests. I only do short and simple ones such as the one found on this website (http://drawahouse.com/TakeTheTest/) where seniors draw a house. There are some that can not use writing utensils at their stage of memory loss, so a staff member/volunteer will do the work for them and include what the senior says to include. Based on what they include in the drawing, it says certain things about their personality. I only highlight the positive traits as that is important when working with dementia patients.
I mentioned this in another topic, but I led a Vegas-themed mystery where staff acted and seniors guessed who the thief was. I can share this with you if you are interested. The seniors loved it and got really into it!
Music can be a great reminiscing tool, so we play games like "Name that Tune" where we only play part of a song and they have to guess it. Many are very good at this! I hope these ideas help. I can provide more if needed.