Sorry for being a one trick pony on this subject but you may like to take the treasure basket to the next stage and try heuristic play for toddlers: (or other people reading the thread might so I am going to stick it on even so) :
"Older children who can crawl still enjoy the Treasure Basket activity, however they may be ready to move on to the next stage of discovery, Heuristic Play with Objects. Large collections of natural and household objects are used, for instance, 100 curtain rings, 20 cardboard tubes, 10 mug trees. The children interact with the materials in whatever ways they choose, learning about cause and effect as they roll, stack, fill, sort and transport the objects. Because a large number of each object is provided, sharing issues are avoided and even a group of possessive toddlers can play side by side with little or no conflict. Elinor Goldschmied recommends this type of play for those up to 2 years old, but I have found that it intrigues children up to the age of 3 and beyond (although banging the materials and running about with them does tend to replace some of the more thoughtful endeavour at this age).
The materials are stored in drawstring bags, and tidying up at the end of the session can form an agreeable part of the activity. This sort of play takes dedication to recreate at home, but is strangely addictive once you begin collecting, and is a marvellous distraction for either individuals or groups of children on rainy days. Suggested items include:
Jar lids Bunches of keys
Wooden pegs Drinking straws
Curtain rings Seashells
Ribbons Ping pong balls.
You will need ideally at least 50 of each item plus at least 10 each of your choice of the following containers:
Large tins Straw baskets
Cardboard tubes Mug trees and kitchen roll holders
Wooden or bamboo bowls Wooden or cardboard boxes.
Choose an appealing combination of objects for each session. For a single child, the baskets, straws, shells and curtain rings might provide ample resources for experimentation, but for a group you would need say 2 or 3 sets of containers, and 4 or 5 of the other sets of objects. Balls and tubes are always a popular combo, as are curtain rings or bracelets and mug trees. You will no doubt devise your own favourites."
(from the guide to heuristic play I wrote for my group)
I would recommend ping pong balls and cardboard tubes to start with - 144 ping pong balls on eBay atm for £12 inc p + p - cardboard tubes are free if you ask your friends to save them for you. Not much for a very adaptable and long lasting play activity that 1, 2 or 3 toddlers can enjoy together.