I originally planned to rent a car as well to visit Uppsala, but since we would be eating, drinking, etc with friends, I opted for the high speed SJ trains (34mins) from Stockholm Central to Uppsala Central at ~ $8/person each way. Downloading and using the SJ Train app is highly recommended if you use the trains in Sweden as the app accepts US (non-European) credit cards and gives you all the live train info. Plenty of bikes and E-scooters available to rent via Uber app, Bolt App, etc, if you need to get to/from the train stations quicker, plus plenty of taxis in both cities.