You'll need a fair amount of space for the running and rolling part. For very young children, have the leader simply shout out the commands Reach Rub Run and Roll in order. When the leader says RUB, the kids rub their tummies like they just ate something yummy. When the leader says RUN, the kids run around the room. When the leader says ROLL, the kids drop to the ground and roll around on the floor.
Relay races are another good letter R activity.
Using Rice to fill in a picture is fun and adds texture. Use rice to fill in or make a frame for the letter R-r template to help reinforce the letter shapes.