Back in the day, how I used to get from LGA-EWR.

Walk from crash pad to USAir shuttle terminal
JS LGA-BOS
Walk to Continental terminal
JS BOS-EWR.
Total time in transit, 4hrs
Cost: free
Usually a seat in First so well rested too.
Disclaimer - you need about 8hrs notice and reasonable weather to pull it off (no ground stops.)​​​​​
Note, this was back when there were hourly flights in both directions. Not sure if that still exists today