J.G.
Maybe get old boxes that the kids can fit themselves into. Tie string to it for shoulder straps. Let them decorate their 'car' and let them run around the backyard choo-chooing ??
Buy conductor hats online and get markers and let the kids decorate their hats? Or just buy regular hats and let them wear them.
Get toy trains and hide them around the back yard. Do an 'easter egg' type of hunt, hunting for the trains. (Or hide tracks, then they can put the tracks together once they find them all).