I've been doing this for a year and being lazy have still got a fair few sign ups to do. What I would suggest is the no risk casino or the low risk casino reloads. Specifically free spins offers even if they do come with wagering requirements because it's often easy money that you could be missing out on. The low risk in my mind are the wager £5 for fs, wager £10 for fs and the wager £20 for fs. Paddy Power and betfair have a lot of these offers and they can be incredibly lucrative, if you're lucky it's an easy £5 or £100 or more. There are also free spins ladders on PP and BF but I'd advise only doing the first rung or 2 as these can be profit killers UNLESS you're doing well on the first couple of tiers. William Hills games offers come with horrendous wagering but I've made good money off those, their Macau offers tend to have no wagering on the fs winnings. I even managed to win a very small jackpot on Macau, so they're worth doing!
Also worth looking at are the casino and bingo sign ups that tend to come with fairly low wagering requirements in exchange for free spins or a bonus and obviously bingo tickets for the bingo sign ups. I made over £160 off one bingo sign up and there are frequent reloads. Heart Bingo comes with a daily free game which is worth doing.
In relation to sports offers, Ladbrokes do the acca for free spins offer on footie, which can be good value. Paddy and Bet365 do 2up which if you hit it can be incredibly lucrative. Plus bet365 dish out bet credits if you bet with them regularly, and also do the bet and get on big sporting events. William Hill are doing the Golden Goal offer regularly which is an easy £30 odd if you back the correct team. This offer is +EV even if you don't hit it every week. For 2ups you don't need to do massive stakes if you're not comfortable with doing so.
Due to the international break most of my profit has come from slots offers. I do find slots boring but profit is profit! Hope this is helpful.