Depends on what you want to learn. Rocketpool has a testnet setup so you can play with ETH staking: https://docs.rocketpool.net/guides/testnet/overview.html

Get some free testnet ETH and try it out!

I haven't looked into minting NFT on testnet. You can write your own contracts on testnet tho but that's a heavier lift.

Disclosure: I own crypto and think ETH will be <$0.05 USD for a transfer within 4 years.

Depends, a non technical friend of mine asked me about polygon because their favorite artist was mining NFT's. The coinbase -> metamask -> mint process was less than .25cents