Infants learn best by moving around and refining their motor skills. So, a good way to encourage this in children over 3 months or so is to make a small course for them to go through. This should not be overly complicated; just put a few things in their path, like stuffed animals or toys, then ask them to crawl over to you, navigating through and around the obstacles. This will help improve their ability to creatively problem-solve.
Infants are at an age where they are exploring and learning how to understand their senses. So, you should give them activities where they can do this in a wide variety of ways. Try giving them a few different textures to play with, such as a piece of tape, a piece of paper and a sample piece of carpet, so that they can feel the contrasts between them. For hearing, you can let them bang on different materials to hear the different noises they can make. The same goes for the other senses; focus on letting them explore contrasts on their own rather than just showing them to the infants.
Toddlers are able to walk a little bit, and are even starting to climb. So, you can build them a small climbing activity to play on. Even better, if the toddlers are two or three, they can help you a little by moving cushions from a couch, and helping you with the design of the structure. The act of helping you will encourage their creative thinking skills, while the complex movements required for climbing will stimulate their physical creativity and problem-solving skills.
Toddlers are at the age where they can benefit from creating things for themselves. According to Baby Name Box, a good activity is to make things out of edible clay, which you can make with half a cup each of peanut butter, honey and non-fat milk solids and 1 and 1/4 cup of graham cracker crumbs. Give some of the mixture to each child and ask him to make whatever he wants. Since they are toddlers, you will have to provide them with some guidance, asking leading questions about what they want to make and how they are going to make it. You can give suggestions, but you should try to make it as much about them as possible; they should be making something from their own ideas.