Using the Litter Box mode is your best bet. You don't have objectives or health to maintain, and you can exit through the menu any time you want without losing your item count.

The level I like to repeat the most is the Supermarkat. It has over 3,000 items, though I didn't count the shopping cart contents since they were time intensive to get.

Since you don't need to worry about health, go for cats with the best balance of the other 3 stats that suits your play style. I liked Catzilla and Lyn the best since they had the best balance of the other three stats with emphasis in swatting.

For the majority of the shelves, all you have to do is run down the back of the aisle to get them to fall. START FROM BOTTOM SHELF TO TOP SHELF. This makes sure that there is actually floor to fall on since items pile up quickly. For light to medium items, either touching the edge of items or headbutting them directly as you run works great. This works for heavy items, but you may need to knock the first one out of the way for yourself first. For really heavy items on the outer walls you will need to swat manually.

Good luck!