As it is summer now, I think now is the best time to talk about keeping your bunny cool. For most house rabbit owners this isn't much of a problem: you're bun gets to enjoy the air conditioning with you! (side note: if you do have air-conditioning, make sure you're bun is never confined directly over or under a vent where they can actually become too cold.)
In my case, I happen to not have air-conditioning (besides a window air-conditioner which is really ineffective) at my mom's house where I live during the summer (my apartment does). So for those in a similar situation here is a few tips to avoid a hot bun :
1) Frozen water bottle- Pretty self-explanatory, I recommend removing the labels so that they aren't eaten by your bunny, also its a good idea to keep a couple extra in the freezer so that you can switch them out and refreeze as they thaw. This is a pretty common practice at fairs and for those who keep rabbits in outdoor hutches, apparently Umbra has never heard of it though because she seems to not understand that she is supposed to lay next to the bottle to cool off. Instead she just tries to chew on them.
2) Keep a fan nearby to help circulate the air. Like any electronic make sure the wire is chew-proof. Also make sure your bun is okay with the fan. Some rabbits don't like the noise or the wind created by fans.
3) Wet their ears: since rabbit's can't sweat to cool off like humans do, they release heat through their ears. The blood vessels in the ears swell to accommodate extra blood flow and the thin skin of the ears allow heat to pass through. You can help this process by rubbing water on those big ears.
4) Take shelter in the basement. Normally it's too cold in a basement for a rabbit, but on really hot days I will move Umbra downstairs to escape the heat. Like any room in your house, make sure that it is bunny-proofed. If your basement is unfinished or partially finished, it may be an unsafe place to keep you're bun.
Here's hoping you have a happy, mild and safe summer with your bun!