Upvote:3
Parking in central Amsterdam is scarce and expensive.
Street parking is often restricted to badge holders and even then hard to find a spot near the house.
If you rent from a down town location, best ask them if they allow you to park overnight (or whatever hours) in their parking facilities. If they use a multi story car park which also allows people to just pull in, (not just long term users,) there is no reason not to use it.
Otherwise, you may want to consider the park and ride from one of the edge of city locations, there are several around the city, as it is a relatively cheap option with free public transport into the city. I do not know how well it will work overnight, as I have never needed for that.
Upvote:4
The parking garage nearest Sixt at Central Station costs € 5 per 50 minutes, or € 55 per day. Other options downtown have similar rates.
This site (in Dutch) has an overview of all parking garages in Amsterdam.
The cheapest, and IMO most convenient option would be to use a P+R (Park and Ride). These are located outside the city center, but near public transport hubs. If you use public transport to go downtown, rates are € 8 per day, or as little as € 1 per day if you arrive after 10 AM. Otherwise normal rates of € 1.40 per hour apply. More details.
This is an index page containing all information about parking in Amsterdam.