Pick one small protocol idea (a vault, a time-lock, a tiny AMM, a per-user counter). Apply the toy-first methodology end-to-end on either Ethereum (Foundry + Anvil) or Solana (Anchor + localnet): write the load-bearing mechanism in under 200 lines, define one invariant, write a property test that catches a deliberate violation, then run it against a forked or local chain. Write a 1-page note on what the toy taught you about the chain you chose, and what would change if you ported it to the other chain.