It has been over a decade since my I've taken the CTA and I am confused by my options from ORD.

I need to go from ORD downtown to the Navy Pier area (requiring one bus transfer} then return on Sunday.

I am trying to understand my options
1. Buy single ride Ventra ticket for $5 which includes the bus transfer, then buy another ticket with transfer ($2.50 cash only?) on the bus back? I'm not clear what I can but from the driver and my options to pay
2. Buy Ventra card for $5, load $2.50. Can I register immediately to use the $5? Then save the 2.50 for the return? I could of course load $5, register later, but the I'll have a $2.50 balance and probably lose that due to inactivity
3. Pay with phone costing $12 (5+2.25+2.25+2.50)
4. Buy card with app somehow?