Blue tongue skink immediately came to mind for me too, those guys are super mellow.
Rats IMO are definitely not a good choice if you want something low energy and cheap. Females are *constantly* on the move and while males are supposed to be more relaxed, none of mine ever wanted to sit still either. It's a total crapshoot whether you get lucky and end up with a male who'll tolerate sitting in one place for any length of time. Rats can also have a lot of health issues that rack up a lot of money in vet bills over time, even with top quality care. Myco flareups, respiratory infection, and tumors are all extremely common. One of my females developed a maloclusion of her incisors and I had to bring her in to the vet every 2 weeks or so to clip her teeth because I was too nervous to try it myself, and I've had multiple rats die in surgery, or just turn up dead in the cage of mysterious causes after showing no signs of ill health.