5 nights in Reykjavik is not ideal- most people will do at least 1-2 nights further out to give you better access to more variety. Having said that, there is quite a bit you can explore from the city on a day trip, including most of the most famous touristic sites.

If it’s a special trip to Iceland I would avoid using the certificate. If you’re just looking for a fun week without any hotel costs, then you could do worse.