You can let the node continue to run, which secures the Bitcoin network by confirming and validating Bitcoin transactions and blocks. You could use it as a wallet. You can enable JSON-RPC and build an entire Bitcoin service (exchange, buy/sell Bitcoin, etc) around it with whatever programming language you wish.

Back in the day, running a full node was how to CPU mine, which was later removed due to difficulty and profitability. I started with CPU mining on a full node when the blockchain wasn't even 100 MB and could be downloaded in a few minutes. The good old days. The amount of BTC you could mine back then in a week with crappy GPUs now takes years with the latest ASIC.