We suggest to buy the 9 Euro ticket, a flat rate ticket on all means of local/regional public transport (including tram and bus in Hannover): https://www.bahn.com/en/offers/regional/9-euro-ticket-en. It is valid for the complete month of August.
Current COVID Rules
Entering Leibniz University is only permitted upon compliance with the 3G rule (more information here). At present there is no mask mandate in the buildings, but you are welcome to wear a mask. Please keep in mind the continuing mask mandate in German public transport.
You can find information on sightseeing in Hannover here: https://www.hannover.de/en. We strongly recommend the visit of Herrenhäuser Gärten (especially the Berggarten)