Hi,
You can use a free plan for this purpose and limit it to days, as this version does not yet have an automatic feature, and the user will need to select Select Plan on the free plan (in this case, users will know that this plan is limited in time, and also what options are included).
Also, if you want to assign a free plan after registration automatically, please see this similar topic: How can I set a default membership plan during registration?
I hope this is helpful to you