I have some unused pins, should I connect them together and to the ground or leave them floating?
The simplest thing to do is set them to INPUT_PULLUP. If you want a purely hardware solution then you could connect them to GND through some resistors (10kΩ or whatever you have in that kind of order of magnitude will do).
You don't want to just tie them directly to ground, and you don't want to link them directly together - that could lead to short circuits and dead IO ports if any of the pins happened to get set as output and HIGH in software.
Leaving them floating in most cases won't do any harm. Even touching the pins won't destroy anything. Tying extra external hardware is just unnecessary (though a 10k resistor would be harmless as well). Of course, you could also think about potential extra usages for those pins if you want to make your design a bit "future-proof". For example, adding 4k7 pull-up resistors to A4 and A5 and a connector could allow you to use the I2C interface for additional functions.