From a previous mouse owner, two or 3 female mice would be a great 'step up'. Just make very sure with the seller that they are definitely females! You get some stunning patterns/colour varietals and they're so pretty. Very little work, and with a quiet environment and regular handling will get quite tame over time.

Once you feel comfortable with the idea of breeding, you can separate a tank and have one of the females breed. You'll need to make her a nursery, and remove the male after a couple of days. You can get some stunning little mice that are adorable, depending on what colours you blend from the parents. It's always a suprise when they get their fur and you finally see what colours have been carried on. Breeding even one litter is a big committment though, so please take it on very carefully. It's extra work and while I felt rewarded, you need to take the welfare of mom and babies seriously. Go to fancymicebreeders.com and you'll find all the info you need there. Great site, so register if you want and you can ask all the 'mouseperts' the questions! Very cheap to own, and you can watch them for hours, the stuff they get up to. Each mouse has an individual kind of personality, so a step up from fish. Start off with two females of different patterns that you like, and see how it goes. When you're sure, divide the cage and allow one to breed. At this point the male must go back to the pet shop. NEVER have two 'bucks' (males in the same cage/tank) or they will fight and it can get gruesome. I would recommend a large tank that is split down the middle. So that you can breed if you want to. A removable divider is great, so that your females can have more space when not preggers. It's a lot of fun, and cheap.

Clean the cage out thoroughly twice a week with detergent (get all the corners especially) and rinse thorougly, and the cage will smell like clean sawdust, it won't stink or anything. They keep themselves very clean and will only use one corner of the cage for their ablutions. They will use the corner furtherest from their food for that. You can use ripped up toilet paper for bedding on top of the sawdust, they love that. They will need a place to 'crawl into' though, so spend the money on a small plastic 'mouse house'. Once again, not very expensive and you'll use it forever basically.

The babies are beyond cute, they look like tiny tiny puppies. It's a big responsibility though, so take it seriously. All the pet shops I've been to will buy back any mice that are 'surplus' or just too many in that space. You won't get a lot of money back, but it goes towards your outlay and it's a good hobby. I used to watch mine for hours. It was like Melrose Place, just with mice. LOL. Do check out that site. I got lovely ones with badger stripes, ones with dalmation spots, etc. It was awesome.