These are the types of requests that make a Marketo administrator's heart go pitter-pat with joy.
Data cleansing is a great idea - just going into Marketo's system list of suspected duplicates and seeing if there is merging you can do there (with your admin's permission) is so helpful. Also consider building some smart lists that your teams might find useful in the future - by product interest, geography, etc. Whatever your business might find useful.
Along similar lines, you could use smart lists to see if certain data is not available or "dirty" and put together some plans to clean the data up. Do you have a number of leads where United States is spelled U.S.A. and you need it as "US"? First run some smart lists to see and then see where those leads come from (your CRM? List imports?). You could then build some basic data management campaigns to change "bad" values for United States to the one you want. (Check with your CRM admin to see if Marketo has permission to change this data value if the source is your CRM.) You can do similar searches to look for inconsistent values for job title, state, market, etc. and then build data management campaigns to correct those errors as they come in. (Again, checking with your CRM admin if data is coming from that direction.)
Period costs are underutilized and can be so important, so I will strongly second your suggestion there!