If I build my spawning area two blocks high with trapdoors on the ceiling, most other mobs (skeletons, witches, endermen, zombies) won't spawn. However, spiders do still spawn and they are annoying.
If I put a carpet down in the corner square of every 3x3 area of floor, that does stop spiders spawning but I was intending to use a flying machine to sweep the creepers to their doom and the carpet stops the machine. It would also mean I couldn't wash the creepers away as the carpets would also wash away.
I tried half slabs but that didn't prevent spiders spawning.
Does anyone have a suggestion of something that will not stick to a slime block that I could use to prevent spiders spawning?
I haven't seen that so I shall have a look but I did try using cats but it didn't work very well. For some reason the creepers weren't reliably afraid of the cats.
While I was building the test farm, I made an AFK station that was accessible from the farm and made one of my cats sit by my feet. I got blown up several times by bold creepers that didn't seem at all bothered about the cat.
The scaredy cat farms seem to work very well for me. Sure you'll get some creepers that don't respond to the cat right away, and maybe even a few who'll run around in circles instead of exiting the spawn area, but all in all you should get loads of creepers and gunpowder. I tested one that was roughly the same size as the piston based one I built in my SSP world and the scaredy cat farm was better.
Otherwise, you can imbed fence posts in the floor and water flushing should still work. Put the fence posts where you would normally put carpet or slabs, and put them IN the floor, not on it. Wooden fences, netherbrick fences or cobblestone walls should all work.
Mr_N_Derman's idea about path blocks is an interesting one. Might work, as the last time I test it, water would flow over tilled farmland without changing it. So tilled farmland or path blocks are other alternatives for a flushing farms. Hmm... thinking about that a bit more, mobs jump in water, so they might break farmland. Path blocks may still work, though. So fence posts or possibly path blocks should give you a spider free flushing creeper farm.
I tested these in my spider test world. The path blocks and fence posts both failed to prevent spider spawning (with a block in the corner of each 3x3 square) but the cobblestone wall did. The fact that cobblestone wall and fences are different surprises me but as soon as I replaced the cobblestone walls with fences, spiders started spawning immediately.
It looks like a flushing farm should work well. I have to admit that I'm slightly disappointed not to have found something that doesn't stick to slime blocks. I've just finished a wither skeleton farm that I've been playing around with for ages but never really satisfied with it. I installed sweeping arms made out of observer block flying machines and it works really well. I've made lots of flushing farms (because it's a very effective mechanism) and I'd like to try something a bit different.
Might check out Ilmango's lag-friendly creeper only farm. I built a version of it on our server, and it works great. It's fairly expensive, but very light on server load.
Yup, that one works well enough. I have a very slightly modified version of that added to my iron farm in my older SSP world.
The creeper spawning mechanism occupies a 16x16x16 volume inside that creeper face facade, so it is smaller than the ones in ilmango's video, but it still gives me more gunpowder than I can use. I did get more spawn blocks in the limited space by eliminating the gap between floors.
I know ilmango said that you would have problems with quasi-powering nearby pistons without the gap, but I experimented with that and found it really wasn't much of a problem. I occasionally get a pair of pistons stuck closed, but they self-clear after a few minutes, and without the gap I was able to fit an entire extra layer (4 additional rows of spawn blocks) in the same space. The tradeoff was, either temporarily lose the occasional pair of spawn blocks, or have 64 less spawn blocks permanently. So I built it with the layers stacked directly on top of each other and it has been working great.
In my testing the scaredy cat farm is even better, but the ilmango farm gives me more gunpowder than I need, and it didn't require cats. I had plenty of iron, string and wood (iron farm, spider farm and tree farm), but I didn't have cats, and the nearest junge was very far away. Even though I had the materials on hand, it was still a lot of crafting.
For my small ilmango farm I needed to craft:
5 stacks of sticky pistons
5 stacks of tripwire hooks
5 stacks of iron trapdoors
5 stacks of cobblestone walls, and
5 stacks of stone slabs
[whew!] That was a lot of crafting. I built this before there was a recipe book. Nowadays that would be so much easier.
On top of that I needed 10 stacks of string and 10 stacks of building blocks, and that's all just for the inside!