Mushroom island near spawn
Java Edition 26.3. Checked with Seeder's engine on .
In the muuuuushroom islands biome, hostile mobs can't spawn. Build your base there if you want some peace when you're not out in the world doing important stuff.
The game never lets you spawn on the island itself, so every seed here tells you how far it is and which way to go to find the island. They're all between 200 and 400 blocks away.

1 A mushroom island about 250 blocks across, 253 blocks south-east of spawn.
Seed-7078951977818456085
What X, Z Distance Mushroom Fields 2, -110 253 blocks Shipwreck -272, -80 208 blocks Trial Chamber 16, -304 210 blocks Ruined PortalUndergroundAir pocket 32, -464 295 blocks Abandoned Camp -416, -464 295 blocks Ancient City -272, -624 361 blocks Stronghold -1548, -572 1.4k blocks Spawn in a grove at -192, -272, around Y 90.

2 A mushroom island about 850 blocks across, 233 blocks south-west of spawn.
Seed-4291225404178966385
What X, Z Distance Mushroom Fields 230, -394 233 blocks Shipwreck 384, -512 86 blocks Ruined PortalGiantOcean portal 192, -512 179 blocks Abandoned Camp 608, -576 256 blocks IglooWith basement 624, -848 374 blocks Stronghold 1956, -1820 2.0k blocks Spawn in a forest at 352, -592, around Y 77.

3 A mushroom island about 760 blocks across, 277 blocks west of spawn.
Seed-963374613974828980
What X, Z Distance Mushroom Fields 46, 426 277 blocks Ruined PortalOcean portal 304, 352 117 blocks Shipwreck 176, 432 148 blocks Trial Chamber 192, 256 248 blocks Monument 192, 160 334 blocks Stronghold -92, 1300 928 blocks Spawn in a taiga at 320, 468, around Y 65.

4 A mushroom island about 280 blocks across, 222 blocks north-west of spawn.
Seed-5222399596891977076
What X, Z Distance Mushroom Fields 38, -14 222 blocks Trial Chamber 96, 128 112 blocks Ruined PortalUndergroundAir pocket 304, 224 136 blocks Shipwreck 0, 240 236 blocks Abandoned Camp 192, 416 288 blocks Monument 80, -176 330 blocks Outpost 512, 288 344 blocks Stronghold 68, 1652 1.5k blocks Spawn in a savanna at 208, 128, around Y 80.

5 A mushroom island about 370 blocks across, 303 blocks south of spawn.
Seed5752332856810546851
What X, Z Distance Mushroom Fields 114, -114 303 blocks Jungle Pyramid 96, -448 58 blocks Trial Chamber 96, -384 58 blocks Shipwreck 96, -224 198 blocks Ruined PortalJungle portal 112, -640 226 blocks Trail Ruin 256, -688 294 blocks Abandoned Camp 448, -560 336 blocks Stronghold 1412, -892 1.4k blocks Spawn in a jungle at 144, -416, around Y 72.

6 A mushroom island about 160 blocks across, 298 blocks north-east of spawn.
Seed7538941640283757942
What X, Z Distance Mushroom Fields -242, 430 298 blocks Trail Ruin -384, 784 116 blocks Ruined PortalGiantJungle portal -512, 784 148 blocks Shipwreck -144, 672 272 blocks Trial Chamber -224, 880 283 blocks Stronghold -44, 1652 1.0k blocks Spawn in a stony shore at -416, 672, around Y 65.

7 A mushroom island about 140 blocks across, 270 blocks north of spawn.
Seed-8531640712777030001
What X, Z Distance Mushroom Fields -26, 82 270 blocks Shipwreck -96, 240 138 blocks Trial Chamber 96, 272 138 blocks Ruined PortalUndergroundAir pocket 320, 384 338 blocks Monument -288, 64 396 blocks Stronghold -924, 1492 1.5k blocks Spawn in a dark forest at -16, 352, around Y 66.

8 A mushroom island about 420 blocks across, 342 blocks south-east of spawn.
Seed5096040350616420880
What X, Z Distance Mushroom Fields 74, 34 342 blocks Outpost -224, -240 82 blocks Ruined PortalUndergroundAir pocket 48, -272 279 blocks VillageTaiga village -448, -368 318 blocks Trial Chamber -320, -464 324 blocks Trail Ruin -480, 16 324 blocks Stronghold 996, -908 1.4k blocks Spawn in a plains at -208, -160, around Y 66.

9 A mushroom island about 450 blocks across, 374 blocks north of spawn.
Seed-7222111417440254417
What X, Z Distance Mushroom Fields -358, -22 374 blocks Ruined PortalOcean portal -304, 176 187 blocks Trial Chamber -368, 160 192 blocks VillageZombie villageSnowy village -368, 576 224 blocks Shipwreck -96, 256 288 blocks Igloo -464, 672 334 blocks Stronghold -1068, -924 1.5k blocks Spawn in snowy plains at -368, 352, around Y 91.

10 A mushroom island about 220 blocks across, 252 blocks south-east of spawn.
Seed3178074831328762545
What X, Z Distance Mushroom Fields -358, 554 252 blocks Ruined PortalOcean portal -560, 256 129 blocks Shipwreck -384, 544 226 blocks Trial Chamber -864, 336 324 blocks Stronghold -1628, 532 1.1k blocks Spawn in a desert at -544, 384, around Y 65.
Need more?
These ten seeds are the best out of thousands I searched. Seeder can find as many as you like, given the right premises, with your own range and in your favourite version.
Search in the finderGood to know
- Make a boat before you go, and milk a mooshroom with a bowl for free mushroom stew.
Questions
What is a mushroom island?
A rare biome out in the ocean, covered in mycelium, giant mushrooms and mooshrooms. Minecraft Wiki
Why can't I spawn on it?
The game always picks a spawn point on the mainland. You land on the nearest shore.