Question: My daughter is 8 and has been taking private piano lessons for around 3 months now but will take a break from lessons over the summer. In the fall (when school starts back up) she will be taking cello lessons at school.

What if anything can I do with her over the summer on the piano that will help prepare her or make learning the cello easier come fall?

She is learning to read (both treble and bass clef since she is playing the piano) and I have started doing some ear training games with her, but I am looking for more and am open to suggestions.

The ear training we have been doing is very basic so far, all diatonic, I play I, IV, V to get the key in her ear and then play DO and another note and she has to tell me what note it is. 

(I studied music in school, and play the drums and Guitar, just so you know my background. I do not have any experience with classic instruments like the cello and have never played a fretless string instrument.)

Answer: 
I have a couple of thoughts. First, she will be reading primarily bass clef for cello, so any drilling of those notes in particular will be useful. Also, help her to "forget" her piano fingering. I have piano students who play stringed instruments and discussing fingering is such a pain. I wish there was a universal fingering system! "1" on piano is thumb. "1" on cello is index finger, and so on. You could make or purchase flash cards to help with note reading. You can do a lot of rhythm games as well.
Some of your preparation you will probably want to do off of the piano.

Work with your student on being able to quickly and easily identify letters that come before other letters, and recite the alphabet backwards from G. This will aid in knowing the note names on the finger board and in the sheet music. Normally people are very good at knowing what letter comes after a letter in the alphabet, but are not as good as instantly knowing letters before other letters. This causes a pause in the flow of reading and/or reciting notes on the staff or fingerboard. Working this exercise before can speed up the note learning process.

Many of the lesson systems will start string players with the D Major scale, as the fingering pattern is simpler to start with on the 5th tuned instruments. Working on understanding the note pattern of D major, and how there are half step between B|C and E|F can help with visualizing the cello fingerboard.

Reviewing the chromatic notes, and reviewing how F moves up to F# and C moves up to C# when playing the Major pattern starting on D can help the student see where the notes are when they begin playing the notes of the beginning D Major pieces.

In some cases the beginning instruction for strings starts with finger numbers instead of note names. You can practice switching between "Piano 1st" and "Cello 1st" and other fingers to make a distinction.

On the piano in bass clef you can work on recognition of the notes that the cello strings play. If the student is able to instantly identify the C G D A notes on the bass clef that correspond with the strings, it sets up an easier recognition of the pattern that the notes are played on each string (open string note on a space, then line-space-line notes to the next string). This sets up the ability to determine which string a given note should be played on in beginning tunes.

Ear training is an excellent thing to do. You can work on whole steps and half steps (whole tones and semi tones) as well as thirds and arpeggios to prep finding notes on the cello.

Finally, one of the most important things you can to to prepare is to work on reading and playing rhythms. Playing a bow for the correct length of time may be challenging for some students, so working on counting out note duration, counting out loud while playing, and knowing how many notes are in other notes (four eighth notes in one half note etc.) can help rhythm playing in the beginning.