Oats are a good way to get a rabbit to put on weight without too much sugar. You could also try baby rabbit pellets, which are higher in fat. And alfalfa hay. Just make sure you watch her poops with any new introductions to her diet. If she’s producing extra cecatropes you might need to cut down.

What’s her diet like at the moment? If she’s eating well but is still underweight, there could be an underlying issue.

You could try critical care to bump up her weight.

You can do critical care for a week to help. This is how I made Wick gain weight when we realized he was underweight. Twice a day, Wick got unlimited critical syringe feeding. In the morning and at night, I’d feed him critical care until he didn’t want anymore. He’d hop away with the largest stomach haha. After a week, he was up to normal weight and has maintained it since.

Oats and BOSS (black oil sunflower seeds) are good for weight gain. You’ll want to slowly introduce new things to her diet. Any sudden changes could bring about some unfortunate upsets with the GI system. You’ll want to make sure she’s not putting the weight on TOO fast either. A good steady pace is the way to go. I recommend getting yourself a postal scale (cheaper than a baby scale) to weigh them regularly so that you can monitor her progress. It’s also a good investment for any future rabbits you get, or just for routine “maintenance” to ensure their weight remains in the appropriate range.