you can't have everything at the same time! you don't want to spend anything, you don't want to mine anything, and you don't want to work. but at the same time you want to earn, you want to mine and you want to protect against 51% attack?!!!

that is not possible.
you have to give something to receive something in return.
for example you have to spend your computing power and mine some coin to contribute to its hashing power and make 51% attacks harder.
or you have to invest your money and buy some altcoin to have a share of it to run a PoS node and earn stakes.

you can run a full node of some coin, like bitcoin full node and contribute to its network and increase the decentralization of it a small amount but you won't be earning anything.