There's a lot of options, but a form that's getting popularity recently is the SAFE (similar to a convertible note) which was popularized by its use in Y-Combinator deals.

It allows you to set a valuation cap on the round for those early investors and set a discount for their early participation on whatever the valuation of your round ends up being. It's a great way to postpone the valuation and terms to a later date, especially if you still need to prove out your business model.