A toddler obsessed with diggers does not want a bouncy castle. They want to move dirt, and they want to watch something big do it.
A shallow builders' tray or paddling pool filled with sand or dry soil, with a handful of toy diggers, dumpers and a level, will occupy toddlers longer than any single toy on its own. Add real gravel and small stones as "loads" to move — the loading and dumping is the actual play, not the driving.
Some local construction firms, farm suppliers or plant hire yards will let a small group sit in the cab of a stationary machine, engine off, with an adult present. It is worth one phone call. Watching real machinery move, from a safe distance, beats any decoration you could buy.
Children's hard hats and small hi-vis vests are inexpensive and reusable for every future construction party you throw. They do more for a child's sense of the theme than a paper plate ever will.
Only with the machine off, an adult right there, and permission from whoever owns it. Never near a machine that is running or moving.
The sandpit alone is enough. It is the activity that gets the most actual play time regardless of what else you arrange.
Part of our guide to Birthday party for one obsession.