Slabs of stone, marble or large ceramic floor tiles in a shaded spot - they can lie on them to keep cool.
Trees and bushes give good shade - I try to site the rabbit runs under overhanging branches where the soil is naturally cooler and they don't get direct sun in Summer.
Provide additional shade over any open area, but not restricting air flow - damp down if needed during the hottest part of the day to increase heat loss by evaporation.
You can get an ice pod version of microwave heat pads. Be careful that there's nothing toxic in them if they get chewed. The contents of standard freezer blocks may not be safe, so it depends on the rabbits. You could always put them under a thin stone slab / large ceramic floor tile.
Bring them inside to a shaded room if conditions outside are extreme.
If it gets really warm and they are struggling, I would shear them (or partly shear them). It also means that you can keep an eye on other potential issues, eg flystrike. Keeping them well groomed will help them to regulate their temperature to some degree (ie matted fur will be more insulating), but a huge coat can be a health risk if they can't cool down naturally.