She could put you down for a short nap, read you a story, give you a pacifier? There's also those amazing activity books where the page is full of items and the parent should tell the child which item to search for, I feel like that could be fun. You could play hide and seek, find out if there's a not-super-busy park near you where you could go, have a tea party for your stuffies, do puzzles, make up silly stories, sing songs, bake cookies together, play dress-up, learn a silly dance, do finger-painting, get a children's science playset and do some neat experiments, paint your own puzzle and later assemble it (there's art stores where you can buy blank puzzle cutouts to decorate yourself, it's pretty great), build a blanket fort, have a pillow fight inside (or outside) that blanket fort, watch a movie or series or something, and have your girlfriend make sure you stay hydrated if you're a hyperactive Little like me who forgets to drink. There's endless amounts of possibilities!
Depending on what age you regress to, there's going to be different activities that will appeal to you. Maybe you could even look up something like "activities for age (blank)" and see if anything sounds interesting? I really hope you have fun, this is a great opportunity^^
