Little fur covered animals ( and I will admit my guinea-pigs have more fur than sense) need extra TLC in the hot weather.
Shade
A nice wafty breeze but not a draught
Somewhere to hide if they feel the need.......but not overheat. Mine still like to hide in the hay 
even though their hutches are enclosed (inside the Pighouse)
Foxes- I've noticed a young adult in the garden in the daytime, bold as brass .
Cats sitting at the run
Poisonous plants n the lawn, springing up overnight.
Flies - the risk of flystrike especially older or overweight animals. Keep a check on poohs and bottoms , the risk of loose pooh is higher with lush grass. As the the risk of bloat.
Shade over the run - a sheet or towel is good. If its really hot wring a towel out in water and put it over the bars
Wasps and bees. A sting can kill a small animal
Water - though mine won't drink it its there on offer and we rinse the salad leaves to leave a bit of water on them.
Minimal hay - just enough for eating , in a rack or loo roll tubes. Mine have shredded card to sleep in for hot weather.
Floor tile to sit on
We keep 2 litre lemonade bottles full of water in our freezer , wrap in newspaper and put in the hutches.
Ours love cucumber and watermelon 
in Summer
Keep an eye out for heatstroke , it can kill. The animal literally pancakes , rapid breathing. You need to cool them , its an emergency situation.
My guineas are all 4yo so not old but a bit middle aged - personally I find it harder keeping them safe in Summer than Winter.
But they're looking forward to a nice few months of grazing , being out till 11pm ( dirty stop outs) and laughing at me as I try to move their runs and peg them down .
At least they mow the grass and fertilise it