One that got mentioned a lot around 2008-2010 but didn't get picked up because Wall Street runs both parties:
Institute a universal Financial Transactions Tax. Nice and small, say 1% or something. Every time a financial asset is flipped (stock sale, bond sale, currency speculation, crypto, etc.), it's taxed on the amount sold regardless of profit/loss.
The biggest sector in the economy now is people making money off of economic rents - transations that don't actually contribute anything or create a product, but simply attempt to profit off of inefficiencies in the system, market manipulation, and the advantages of owning tons of capital. You start taxing all those unnecessary transactions and suddenly they'll have to limit themselves to waiting for actually profitable opportunities that make sense instead of trying to game fluctuations on a day-to-day basis.
If someone wanted to invest in a stock that they think will bring a real profit over a few years, if they want to buy some crypto and then HODL, then the one-time 1% tax will be miniscule compared to the gains they're working for. The 1% only impacts the people who are trying to constantly flip shyt and gamble and who thereby introduce a lot of the fluctuations and problems in the system that fukk other people over.