Mixing 5V Sensors and Devices with 3V3
As far as the LCD display with I2C interface, you need to figure out the Vih minimum to see if that's necessary. And to figure out if the particular MCU you have will tolerate I2C outputs pulled up to 5V. If it's the PCF8574 as Tirdad suggested in a comment you may need a voltage translator since it requires 0.7 * Vcc for a '1'. If it is required you can use the typical MOSFET + resistor level shifter. I would suggest avoiding the automatic bidirectional chips unless you're willing to do enough analysis on drive capability to assure they will actually work reliably.
If the flow meter is open collector you can just pull it up to 3.3V, and if the inputs are 5V tolerant you can use it whether it's 3.3V or 5V.
For the analog input, a voltage divider will work (see your MCU datasheet for guidance on the maximum resistances allowable). However note that if it's ratiometric to the 5V supply you will suffer some accuracy and stability loss if your reference on the MCU is the 3.3V supply.
Note: I didn't touch on protecting inputs that go off the board into the field, but that's an important part of a reliable design.