Immediately, and until you are done patching it, put an orange traffic cone over the hole.
Slip a flat stick tied to a wire in the middle through the hole. Pull the wire up to make the stick cover at least part of the bottom of the hole - you can perhaps twist the wire on the rusty rebar to hold it in place. Push the remaining ends of the wire down below sidewalk level.
Wet the sides of the hole.
Mix a VERY stiff (relatively dry) mortar or cement patch mix and place in the hole. For best results, don't try to fill it level at first - some will drip out the remaining hole at the bottom unless your stick is a very close fit. The aim of the first batch is to really plug the hole. When that has set up a bit, but before very long (a couple of hours) come back and use a slightly wetter mix to fill to sidewalk level.
Alternatively (depending on the size of this tunnel and the expense of "collapsing it next year") call up a company that "pumps grout" and have them show up with truck, poke a nozzle in the hole, and fill the tunnel with "grout" (more or less low-grade concrete, though there's probably other technical differences.) Depending who does what in your area, "cellular concrete" might be another option (effectively "foam cement.")
If it's your tunnel (I presume if you plan to collapse it next year), I'd be just as concerned about the vehicles parked on top of it breaking through (given the obviously deteriorated condition of the sidewalk, and something parked right on top of it in your picture) as of people tripping, liability-wise.