Most "fun with fractions" activities involve giving students something they can get their hands on, like toys or even candy. For example, by giving students a small chocolate bar that naturally snaps into smaller sections, you can teach students the basis of fractions ---
splitting a whole unit into smaller parts. If you do go down this route, however, it is important to make sure no student has any allergies or health problems that involve your material and to use only a relatively small amount, since the sugar in candy can make some students overactive.
A "fun with fractions" activity needs to have a fairly tight structure in terms of what students are doing from minute to minute. Because this type of activity may involve things like toys or candy, it can be very easy for students' hands and minds to begin to wander if they aren't constantly being told what to do next. Each section of the material covered should flow smoothly and quickly from instruction to modeling to student activity, and finally to a check for understanding.
The body of a "fun with fractions" activity should involve students using their materials to give examples of different fractions and solving different fraction-based problems. For example, if you were using something like a clay, you could have students break the clay into four equal pieces and then explain how each piece is a fourth of the whole piece. You could then ask them to lump two of the fourths back together and ask them how much of the clay is in the new lump.
A "fun with fractions" activity should also have a written portion during or after it, even if it is only a short one. This can simply be students recording the problems they go over in activity or it can be their use of the concepts of the activity in solving different problems. Students may not find this portion as fun, but some form of evaluation is necessary to see how well each student has comprehended the material.