Just some suggestions - some of them Sam has done and some he hasn't.
Live off the Land - Sim family cannot hold a regular job. They can only earn money via gardening and fishing. Catching insects and collecting rocks to sell are potential extra sources of income - if you want things to be slightly easier for your Sim.
Starving Artist - Sim may only make money via writing, painting or sculpting. No other sources of income allowed.
The Busker - Sim may only make money by playing an instrument for tips. This choice will be especially difficult because a Sim must reach a certain skill level with their instrument before they can play for tips.
Toddler Mania - Basic idea is that you'll be taking your clone and creating 7 toddlers. Your Sim needs to raise the 7 toddlers to childhood to beat the challenge. This was originally a Sims 2 challenge which has been updated for Sims 3.
Bachelor Challenge - At some point, you'll be exhausted of doing it tough with your clone. This is just a short play thing. Your Sim will be in a house with 7 potential mates. Each day, they should chat to everyone, do one group activity (if you have After Dark - hot tubs are the activity of choice). The Sim with the lowest relationship at noon of the next has to leave the house. By about day 3, your Sim can start individual dating of the remaining residents.
The Asylum Challenge - Your Sim gets locked into a rather small and badly furnished house with 7 residents. You may not control any other Sim. Rules for a Sims 3 version my only note of what I've seen is that the original challenge, you randomly rolled the inmates. I think I'd randomly roll for this one.